Move unit tests to tests/unit

aviftest is a generic encode/decode test.
avifyuv is more a conversion benchmarking tool than a unit test.
Also it will allow to use a separate .clang-tidy config file once all
tests in tests/unit are converted to C++.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 49c0d7c..93602d3 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -5,6 +5,9 @@
 # at once with CMake's ctest command line tool from the build folder.
 enable_testing()
 
+################################################################################
+# General tests and tools
+
 add_executable(aviftest aviftest.c)
 if(AVIF_LOCAL_LIBGAV1)
     set_target_properties(aviftest PROPERTIES LINKER_LANGUAGE "CXX")
@@ -12,20 +15,6 @@
 target_link_libraries(aviftest avif ${AVIF_PLATFORM_LIBRARIES})
 add_test(NAME aviftest COMMAND aviftest ${CMAKE_CURRENT_SOURCE_DIR}/data)
 
-add_executable(avifincrtest avifincrtest.c avifincrtest_helpers.c)
-if(AVIF_LOCAL_LIBGAV1)
-    set_target_properties(avifincrtest PROPERTIES LINKER_LANGUAGE "CXX")
-endif()
-target_link_libraries(avifincrtest avif ${AVIF_PLATFORM_LIBRARIES})
-add_test(NAME avifincrtest COMMAND avifincrtest ${CMAKE_CURRENT_SOURCE_DIR}/data/sofa_grid1x5_420.avif)
-
-add_executable(avifmetadatatest avifmetadatatest.c)
-if(AVIF_LOCAL_LIBGAV1)
-    set_target_properties(avifmetadatatest PROPERTIES LINKER_LANGUAGE "CXX")
-endif()
-target_link_libraries(avifmetadatatest avif ${AVIF_PLATFORM_LIBRARIES})
-add_test(NAME avifmetadatatest COMMAND avifmetadatatest)
-
 add_executable(avifyuv avifyuv.c)
 if(AVIF_LOCAL_LIBGAV1)
     set_target_properties(avifyuv PROPERTIES LINKER_LANGUAGE "CXX")
@@ -35,6 +24,23 @@
     add_test(NAME avifyuv_${AVIFYUV_MODE} COMMAND avifyuv -m ${AVIFYUV_MODE})
 endforeach()
 
+################################################################################
+# Unit tests
+
+add_executable(avifincrtest unit/avifincrtest.c unit/avifincrtest_helpers.c)
+if(AVIF_LOCAL_LIBGAV1)
+    set_target_properties(avifincrtest PROPERTIES LINKER_LANGUAGE "CXX")
+endif()
+target_link_libraries(avifincrtest avif ${AVIF_PLATFORM_LIBRARIES})
+add_test(NAME avifincrtest COMMAND avifincrtest ${CMAKE_CURRENT_SOURCE_DIR}/data/sofa_grid1x5_420.avif)
+
+add_executable(avifmetadatatest unit/avifmetadatatest.c)
+if(AVIF_LOCAL_LIBGAV1)
+    set_target_properties(avifmetadatatest PROPERTIES LINKER_LANGUAGE "CXX")
+endif()
+target_link_libraries(avifmetadatatest avif ${AVIF_PLATFORM_LIBRARIES})
+add_test(NAME avifmetadatatest COMMAND avifmetadatatest)
+
 if(AVIF_ENABLE_GTEST)
     enable_language(CXX)
     set(CMAKE_CXX_STANDARD 11)
@@ -56,12 +62,12 @@
         find_package(GTest REQUIRED)
     endif()
 
-    add_executable(avifgridapitest avifgridapitest.cc aviftest_helpers.cc)
+    add_executable(avifgridapitest unit/avifgridapitest.cc unit/aviftest_helpers.cc)
     target_link_libraries(avifgridapitest avif ${AVIF_PLATFORM_LIBRARIES} ${GTEST_BOTH_LIBRARIES})
     target_include_directories(avifgridapitest PRIVATE ${GTEST_INCLUDE_DIRS})
     add_test(NAME avifgridapitest COMMAND avifgridapitest)
 
-    add_executable(avify4mtest avify4mtest.cc aviftest_helpers.cc)
+    add_executable(avify4mtest unit/avify4mtest.cc unit/aviftest_helpers.cc)
     target_link_libraries(avify4mtest avif avif_apps ${AVIF_PLATFORM_LIBRARIES} ${GTEST_BOTH_LIBRARIES})
     target_include_directories(avify4mtest PRIVATE ${GTEST_INCLUDE_DIRS})
     add_test(NAME avify4mtest COMMAND avify4mtest)
diff --git a/tests/avifgridapitest.cc b/tests/unit/avifgridapitest.cc
similarity index 100%
rename from tests/avifgridapitest.cc
rename to tests/unit/avifgridapitest.cc
diff --git a/tests/avifincrtest.c b/tests/unit/avifincrtest.c
similarity index 100%
rename from tests/avifincrtest.c
rename to tests/unit/avifincrtest.c
diff --git a/tests/avifincrtest_helpers.c b/tests/unit/avifincrtest_helpers.c
similarity index 100%
rename from tests/avifincrtest_helpers.c
rename to tests/unit/avifincrtest_helpers.c
diff --git a/tests/avifincrtest_helpers.h b/tests/unit/avifincrtest_helpers.h
similarity index 100%
rename from tests/avifincrtest_helpers.h
rename to tests/unit/avifincrtest_helpers.h
diff --git a/tests/avifmetadatatest.c b/tests/unit/avifmetadatatest.c
similarity index 100%
rename from tests/avifmetadatatest.c
rename to tests/unit/avifmetadatatest.c
diff --git a/tests/aviftest_helpers.cc b/tests/unit/aviftest_helpers.cc
similarity index 100%
rename from tests/aviftest_helpers.cc
rename to tests/unit/aviftest_helpers.cc
diff --git a/tests/aviftest_helpers.h b/tests/unit/aviftest_helpers.h
similarity index 100%
rename from tests/aviftest_helpers.h
rename to tests/unit/aviftest_helpers.h
diff --git a/tests/avify4mtest.cc b/tests/unit/avify4mtest.cc
similarity index 100%
rename from tests/avify4mtest.cc
rename to tests/unit/avify4mtest.cc