Separate job for tests

ghc-tests were originally tests in GHC's own test suite (not surprising, I guess). They relied on other packages, which made them fragile, so they got moved into head.hackage, where they would always have access to freshly built packages. But at the end of the day, they are GHC tests, unrelated to the question of "Did GHC HEAD break any packages."

Now that the tests are still proving fragile (e.g. ghc#22248, #49), I'd like to let head.hackage pass even if ghc-tests fail. Since we don't have access to the same fragility system in the GHC test suite, I'm going to just split those tests into a separate GitLab job and mark that one allowed-to-fail.

Ideally they just wouldn't be fragile, but in terms of priority I'd rather have CI green first, and then fix fragile tests.