This way, we don't get an abort once the first test gets a warning.
We also can use meson test to run individual tests.
Unfortunately, only ~60% of tests pass.
With autotools the schemas were compiled into each test suite directory
and the tests set GSETTINGS_SCHEMA_DIR to the test build directory.
With meson's gnome.compile_schemas() we can not define a target directory
so just make sure it is built in the gtk directory and set GSETTINGS_SCHEMA_DIR
to the gtk build directory when running the tests.
This makes the gtk+:gtk suite pass when no gtk is installed on the system.