Disable testing on dependencies by default (#2629)

cmake/Modules/AvifExternalProjectUtils.cmake

diff --git a/cmake/Modules/AvifExternalProjectUtils.cmake b/cmake/Modules/AvifExternalProjectUtils.cmake
index 8d19e53..f734408 100644
--- a/cmake/Modules/AvifExternalProjectUtils.cmake
+++ b/cmake/Modules/AvifExternalProjectUtils.cmake
@@ -5,9 +5,12 @@
         # Force static build
         set(BUILD_SHARED_LIBS_ORIG ${BUILD_SHARED_LIBS})
         set(BUILD_SHARED_LIBS OFF CACHE INTERNAL "")
+        set(BUILD_TESTING_ORIG ${BUILD_TESTING})
+        set(BUILD_TESTING OFF CACHE INTERNAL "")
 
         add_subdirectory(${${name}_SOURCE_DIR} ${${name}_BINARY_DIR} EXCLUDE_FROM_ALL)
 
         set(BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS_ORIG} CACHE BOOL "" FORCE)
+        set(BUILD_TESTING ${BUILD_TESTING_ORIG} CACHE BOOL "" FORCE)
     endif()
 endmacro()