Adjust scopes of avifincrtest_helpers dependencies

"gtest/gtest.h" is included by avifincrtest_helpers.cc but not by the
header avifincrtest_helpers.h, so move GTest::GTest to the PRIVATE
scope. Also move avif_enable_warnings to the PRIVATE scope.

Part 3 of the fix for
AOMediaCodec#2340.

tests/CMakeLists.txt

@@ -93,9 +93,11 @@ endmacro()
 if(AVIF_ENABLE_GTEST)
     check_avif_option(AVIF_GTEST TARGET GTest::GTest PKG_NAME GTest)
     add_library(avifincrtest_helpers OBJECT gtest/avifincrtest_helpers.cc)
-    target_link_libraries(avifincrtest_helpers avif GTest::GTest avif_enable_warnings)
+    target_link_libraries(avifincrtest_helpers PUBLIC avif)
+    target_link_libraries(avifincrtest_helpers PRIVATE GTest::GTest avif_enable_warnings)
     add_library(avifincrtest_helpers_internal OBJECT gtest/avifincrtest_helpers.cc)
-    target_link_libraries(avifincrtest_helpers_internal avif_internal GTest::GTest avif_enable_warnings)
+    target_link_libraries(avifincrtest_helpers_internal PUBLIC avif_internal)
+    target_link_libraries(avifincrtest_helpers_internal PRIVATE GTest::GTest avif_enable_warnings)
 endif()
 
 if(AVIF_ENABLE_GTEST)