Explicitly skip TAP tests under Meson if disabled

Explicitly skip TAP tests under Meson if disabled

If the tap_tests option is disabled under Meson, the TAP tests are
currently not registered at all.  But this makes it harder to see what
is going on, why suddently there are fewer tests than before.

Instead, run testwrap with an option that marks the test as skipped.
That way, the total list and count of tests is constant whether the
option is enabled or not.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/ad5ec96d-69ec-317b-a137-367ea5019b61@eisentraut.org