testsuite: Don't pretend we're a GTest

testsuite: Don't pretend we're a GTest

This is a meson test, not a GTest thing. So:

- Use g_print(), not g_test_message
That makes meson test --verbose print the actual log messsages.

- Don't g_assert() all the time
Instead, run tests through to the end and just return a non-0 exit
status.