tests: Ignore stock-id usage warnings

Add ignore_glib_warnings context manager for easily ignoring warnings
caused by stock-id usage. Even though stock-id is deprecated, we still
need to test the related Python binding overrides.
tests: Remove usage of deprecated "schema" property in GSettings creation

Use new and new_with_path instead as the schema property was causing a
hard warning which fails the test suite.
tests: Ignore warnings for GtkAlignment

This was causing unittests to bail. Even though this is deprecated,
we still need to test its usage in the context of pygtkcompat.