This replaces the test removed in 51be7061d025139ba66869d5d99c7157a3ae9edd
It is more principled and tests more critical cases: a crash while parsing.
We need two pieces of plumbing:
- a way to re-enable the crashing #pragmas via a flag, to test parse crashes
- a bit of reshuffling around ASTWorker execution so that we set up the crash handler in both sync/async modes. Sync mode is useful for debugging, so I tested both.