tests: pass test if files have holes; don't write out file if checking fails

[acl-cqc]

• First, refactor: move CompilingHoles out of Error; it is only used in one place, where there is no other possible Error. Make CompileFile return CompilingHoles rather than String.
• Second, pass compilation rather than (expected-)fail if there are holes. This does seem a bit silent, so other options (all a bit harder):
  • Maintain a separate list of files with holes that we expect to fail in this way
  • Do checking first, and only generate a compilation test for files that check without any holes. That way, these compilation tests disappear from the list of run tests, rather than appearing to pass.
• Third, fix the weird bug that files that errord in compilation, still returned an empty bytestring. This is not what I'd have expected from error but the fix seems to be to use evaluate from Control.Exception getting us an IO ByteString that dies if there was an error.

[croyzor]

It'd be nice if we could have a pytest-style "(Skipped)" reporting to be a little bit less silent, but it's not immediately obvious to me how to achive this using tasty

[acl-cqc]

It'd be nice if we could have a pytest-style "(Skipped)" reporting to be a little bit less silent, but it's not immediately obvious to me how to achive this using tasty

testCaseInfo rather than testCase sort of does it - we have to display a message for every testCase using that, which here means every compilation test, but for the passing ones we can use that to hint that there's another step still to do (i.e. validation).

TBH I think a better approach still would be to use testCaseSteps to combine parsing, checking and compilation into a single multi-step test, but that's well beyond the scope of this PR ;)