bug#58065: UNSUPPORTED test cases still can FAIL

[Arsen Arsenović]

Hi,

When a testsuite that uses dg.exp returns ::unsupported:: for some test 
from ${tool}-dg-test or ${tool}-dg-prune, dg-{error,warning} markers in 
that test case are still processed. If the ::unsupported:: is returned 
from ${tool}-dg-test, these markers emit a FAIL, since the output does 
not contain their wanted errors. If the ::unsupported:: is returned from 
${tool}-dg-prune, these markers are processed as normal, which can 
sometimes PASS due to compiler resumption, but can also FAIL. I see no 
other way to report ::unsupported:: status from the dg callbacks.
This feels like an oversight, as it makes conditionally skipping tests 
as UNSUPPORTED nigh impossible.

Previous ML discussion:
https://lists.gnu.org/archive/html/dejagnu/2022-09/msg00000.html

Minimized reproduction case:
https://git.sr.ht/~arsen/dejagnu-demo-testcase/
(commit 4e83a1a0820062387985d866286d73299f0691a6)

Suggested solutions:
- Call ${tool}-dg-prune before processing dg-messages.
- Permit ${tool}-dg-test to return a special value to prevent processing 
dg-messages.

Thanks,
-- 
Arsen Arsenović