[cmake][dune_add_test] Fix cmake guard and skipping tests

If a complicated user defined command was specified, it didn't lead to the dummy executable (that return 77) being run but the user defined command instead. This patch makes sure that the correct command is executed which then marks the test as skipped and not as failed.

changed title from Fix/dune add test cmake guard skip test to [cmake][dune_add_test] Fix cmake guard and skipping tests

changed the description

enabled an automatic merge when the pipeline for d4666b75 succeeds

merged

mentioned in commit 494ea423

On second thought, I want to see that this still skips tests that are usually skipped (I think there is a Vc test...). So I cancelled automatic merging. But otherwise looks good.

assigned to @joe

Hmm. Looks like there aren't any tests that are normally skipped. So I'm gonna merge it like it is.

Besides, it looks like I wasn't quick enough to cancel the merge anyway, and gitlab did not care to update me on that...

mentioned in merge request !404 (merged)

mentioned in commit 98b8155d

mentioned in merge request !406 (merged)

added bug buildsystem labels

@tkoch While infrastructure/issues0#47 (closed) is being worked on, can you try to already figure out what was wrong with your patch and make a new one?

mentioned in issue #101 (closed)

mentioned in merge request !409 (merged)