[dune_add_test] Do not check existance of EXPECT_COMPILE_FAIL targets.

Fixes: #100 (closed). See also: !404 (merged).

Those targets never exist -- instead, cmake is invoked during testing to attempt to build them.

enabled an automatic merge when the pipeline for 468d6951 succeeds

mentioned in commit 6423ed1a

merged

mentioned in merge request !410 (closed)