gck-slot: Improve assertion checks
Make more use of g_assert_cmpint and friends so that the error message indicates more clearly what we were expecting and what we got.
This also has the minor benefit that unlike g_assert(), the more test-oriented macros aren't disabled by G_DISABLE_ASSERT, so we don't need to worry about whether we are relying on their side-effects.
Helps: #42 (closed)