Rename tests/unit to tests/gtest

Because all files under tests/unit will eventually be converted to
GoogleTest and may not all be unit tests.
Move AVIF_ENABLE_COVERAGE closer to its only dependency aviftest.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 9ef98b7..93c4f40 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -6,7 +6,7 @@
 enable_testing()
 
 ################################################################################
-# General tests and tools
+# C tests and tools
 
 add_executable(aviftest aviftest.c)
 if(AVIF_LOCAL_LIBGAV1)
@@ -15,6 +15,21 @@
 target_link_libraries(aviftest avif ${AVIF_PLATFORM_LIBRARIES})
 add_test(NAME aviftest COMMAND aviftest ${CMAKE_CURRENT_SOURCE_DIR}/data)
 
+if(AVIF_ENABLE_COVERAGE)
+    add_custom_target(
+        avif_coverage
+        COMMAND ${CMAKE_COMMAND} -E env "LLVM_PROFILE_FILE=${CMAKE_CURRENT_BINARY_DIR}/aviftest.profraw" $<TARGET_FILE:aviftest>
+                ${CMAKE_CURRENT_SOURCE_DIR}/data
+        COMMAND ${XCRUN} llvm-profdata merge -sparse ${CMAKE_CURRENT_BINARY_DIR}/aviftest.profraw -o
+                ${CMAKE_CURRENT_BINARY_DIR}/aviftest.profdata
+        COMMAND cmake -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/coverage
+        COMMAND ${XCRUN} llvm-cov show $<TARGET_FILE:aviftest> -instr-profile=${CMAKE_CURRENT_BINARY_DIR}/aviftest.profdata
+                -project-title=libavif --format html -output-dir=${CMAKE_CURRENT_BINARY_DIR}/coverage
+        COMMAND echo Coverage report here: ${CMAKE_CURRENT_BINARY_DIR}/coverage/index.html
+        DEPENDS aviftest
+    )
+endif()
+
 add_executable(avifyuv avifyuv.c)
 if(AVIF_LOCAL_LIBGAV1)
     set_target_properties(avifyuv PROPERTIES LINKER_LANGUAGE "CXX")
@@ -25,19 +40,19 @@
 endforeach()
 
 ################################################################################
-# Unit tests
+# GoogleTest
 
-add_library(avifincrtest_helpers OBJECT unit/avifincrtest_helpers.c)
+add_library(avifincrtest_helpers OBJECT gtest/avifincrtest_helpers.c)
 target_link_libraries(avifincrtest_helpers avif ${AVIF_PLATFORM_LIBRARIES})
 
-add_executable(avifincrtest unit/avifincrtest.c)
+add_executable(avifincrtest gtest/avifincrtest.c)
 if(AVIF_LOCAL_LIBGAV1)
     set_target_properties(avifincrtest PROPERTIES LINKER_LANGUAGE "CXX")
 endif()
 target_link_libraries(avifincrtest avifincrtest_helpers)
 add_test(NAME avifincrtest COMMAND avifincrtest ${CMAKE_CURRENT_SOURCE_DIR}/data/sofa_grid1x5_420.avif)
 
-add_executable(avifmetadatatest unit/avifmetadatatest.c)
+add_executable(avifmetadatatest gtest/avifmetadatatest.c)
 if(AVIF_LOCAL_LIBGAV1)
     set_target_properties(avifmetadatatest PROPERTIES LINKER_LANGUAGE "CXX")
 endif()
@@ -65,33 +80,18 @@
         find_package(GTest REQUIRED)
     endif()
 
-    add_library(aviftest_helpers OBJECT unit/aviftest_helpers.cc)
+    add_library(aviftest_helpers OBJECT gtest/aviftest_helpers.cc)
     target_link_libraries(aviftest_helpers avif ${AVIF_PLATFORM_LIBRARIES})
 
-    add_executable(avifgridapitest unit/avifgridapitest.cc)
+    add_executable(avifgridapitest gtest/avifgridapitest.cc)
     target_link_libraries(avifgridapitest aviftest_helpers ${GTEST_BOTH_LIBRARIES})
     target_include_directories(avifgridapitest PRIVATE ${GTEST_INCLUDE_DIRS})
     add_test(NAME avifgridapitest COMMAND avifgridapitest)
 
-    add_executable(avify4mtest unit/avify4mtest.cc)
+    add_executable(avify4mtest gtest/avify4mtest.cc)
     target_link_libraries(avify4mtest aviftest_helpers avif_apps ${GTEST_BOTH_LIBRARIES})
     target_include_directories(avify4mtest PRIVATE ${GTEST_INCLUDE_DIRS})
     add_test(NAME avify4mtest COMMAND avify4mtest)
 else()
     message(STATUS "Most tests are disabled because AVIF_ENABLE_GTEST is OFF.")
 endif()
-
-if(AVIF_ENABLE_COVERAGE)
-    add_custom_target(
-        avif_coverage
-        COMMAND ${CMAKE_COMMAND} -E env "LLVM_PROFILE_FILE=${CMAKE_CURRENT_BINARY_DIR}/aviftest.profraw" $<TARGET_FILE:aviftest>
-                ${CMAKE_CURRENT_SOURCE_DIR}/data
-        COMMAND ${XCRUN} llvm-profdata merge -sparse ${CMAKE_CURRENT_BINARY_DIR}/aviftest.profraw -o
-                ${CMAKE_CURRENT_BINARY_DIR}/aviftest.profdata
-        COMMAND cmake -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/coverage
-        COMMAND ${XCRUN} llvm-cov show $<TARGET_FILE:aviftest> -instr-profile=${CMAKE_CURRENT_BINARY_DIR}/aviftest.profdata
-                -project-title=libavif --format html -output-dir=${CMAKE_CURRENT_BINARY_DIR}/coverage
-        COMMAND echo Coverage report here: ${CMAKE_CURRENT_BINARY_DIR}/coverage/index.html
-        DEPENDS aviftest
-    )
-endif()
diff --git a/tests/unit/avifgridapitest.cc b/tests/gtest/avifgridapitest.cc
similarity index 100%
rename from tests/unit/avifgridapitest.cc
rename to tests/gtest/avifgridapitest.cc
diff --git a/tests/unit/avifincrtest.c b/tests/gtest/avifincrtest.c
similarity index 100%
rename from tests/unit/avifincrtest.c
rename to tests/gtest/avifincrtest.c
diff --git a/tests/unit/avifincrtest_helpers.c b/tests/gtest/avifincrtest_helpers.c
similarity index 100%
rename from tests/unit/avifincrtest_helpers.c
rename to tests/gtest/avifincrtest_helpers.c
diff --git a/tests/unit/avifincrtest_helpers.h b/tests/gtest/avifincrtest_helpers.h
similarity index 100%
rename from tests/unit/avifincrtest_helpers.h
rename to tests/gtest/avifincrtest_helpers.h
diff --git a/tests/unit/avifmetadatatest.c b/tests/gtest/avifmetadatatest.c
similarity index 100%
rename from tests/unit/avifmetadatatest.c
rename to tests/gtest/avifmetadatatest.c
diff --git a/tests/unit/aviftest_helpers.cc b/tests/gtest/aviftest_helpers.cc
similarity index 100%
rename from tests/unit/aviftest_helpers.cc
rename to tests/gtest/aviftest_helpers.cc
diff --git a/tests/unit/aviftest_helpers.h b/tests/gtest/aviftest_helpers.h
similarity index 100%
rename from tests/unit/aviftest_helpers.h
rename to tests/gtest/aviftest_helpers.h
diff --git a/tests/unit/avify4mtest.cc b/tests/gtest/avify4mtest.cc
similarity index 100%
rename from tests/unit/avify4mtest.cc
rename to tests/gtest/avify4mtest.cc