Rerun the OOM `ghc-testsuite` tests individually

Currently, we run ghc-testsuite on CI with -j2 to speed things up. This also means less available memory to individual tests, so for some tests which require much memory or when the yolo mode is on, non-deterministic OOM failures do occur.

The simplest solution is simply removing -j2, but we don't want the CI pipeline to extend for another couple of hours; we should only rerun the individual tests after OOM failure.