test-utils: Add some missing test assertions etc.
tests/test-library uses g_assert_nonnull(), which isn't available in SteamRT 1 'scout'.
While I'm here, add all the other assertion wrappers from GLib 2.58 that aren't available in 2.32, along with g_test_skip().
Detected by !10 (merged). /cc @denittis @jpwhiting