Based on !2216 (merged) (without which the tests added here will fail), please review that first.
gtestutils: Allow failing a test with a printf-style message
This allows a pattern like
g_test_message ("cannot reticulate splines: %s", error->message); g_test_fail ();
to be replaced by the simpler
g_test_fail_message ("cannot reticulate splines: %s", error->message);
with the secondary benefit of making the message available to TAP consumers as part of the "not ok" message.
tests: Make use of g_test_fail_message()
gtestutils: Allow skipping tests with a printf-style message
Forming the g_test_skip() message from printf-style arguments seems common enough to deserve a convenience function.
g_test_incomplete() is mechanically almost equivalent to g_test_skip() (the semantics are different but the implementation is very similar), so give it a similar mechanism for symmetry.
tests: Use g_test_skip_printf()