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.
Copy the location of the test data and binaries from the autotools
build, even though it's not really correct; currently we install the
test data under libexecdir, but it should live under datadir, and we
should use `G_TEST_DIST` to figure it out.
The `state` subdirectory is missing.
