Draft: Improve test diagnostics

• tests: Try to run GLib-based tests under prove(1)

  This gives us better diagnostics on failure, and matches what we do in the pressure-vessel subdirectory and for scripts.

• tests: Run with G_MESSAGES_DEBUG=all if GLib is too old for TAP

  This provides better diagnostic information on failure.

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Mostly untested (perhaps ironically!) but might help when diagnosing test failures, particularly in a non-interactive autobuilder environment.