Failing test exposing async fail-fast deadlock

## Summary This adds a focused regression spec that hangs on current upstream when fail-fast is enabled. To reproduce locally: ``` cabal test sydtest-test:sydtest-test --test-option=--filter="does not deadlock" ``` On upstream master this command hangs for approximately six seconds and ends with: ``` Async runner deadlocked under fail-fast ``` A follow-up pull request will supply the fix so CI records the failure for this commit first.