Explicitly skip TAP tests under Meson if disabled

Under Meson, it is not very easy to see if TAP tests have been enabled 
or disabled, if you rely on the default auto setting.  You either need 
to carefully study the meson setup output, or you notice, what a minute, 
didn't there use to be like 250 tests, not only 80?

I think it would be better if we still registered the TAP tests in Meson 
even if the tap_tests option is disabled, but with a dummy command that 
registers them as skipped.  That way you get a more informative output like

Ok:                 78
Expected Fail:      0
Fail:               0
Unexpected Pass:    0
Skipped:            187
Timeout:            0

which is really a more accurate representation of what the test run 
actually accomplished than "everything Ok".

See attached patch for a possible implementation.  (This uses perl as a 
hard build requirement.  We are planning to do that anyway, but 
obviously other implementations, such as using python, would also be 
possible.)