tests: Make GSETTINGS_SCHEMA_DIR point to the compiled schemas in the build dir

tests: Make GSETTINGS_SCHEMA_DIR point to the compiled schemas in the build dir

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.