Migrate from C++11 to C++14 for tests

GoogleTest requires C++14 starting with version 1.13.

tests/CMakeLists.txt

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 78c2c42..0fd5b1e 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -41,7 +41,7 @@
 
 if(AVIF_ENABLE_GTEST OR AVIF_BUILD_APPS)
     enable_language(CXX)
-    set(CMAKE_CXX_STANDARD 11)
+    set(CMAKE_CXX_STANDARD 14)
     add_library(aviftest_helpers OBJECT gtest/aviftest_helpers.cc)
     target_link_libraries(aviftest_helpers avif_apps)
 endif()