Add a test that ensures tasks get invalidated
This detects a bug unique to the oldest back-deployment targets
for Swift concurrency, where Tasks were getting created with an
incorrect bit pattern that made them immortal.
Verifies that rdar://93087343 has been fixed.
(cherry picked from commit 3c6f6f283a28ee4df608d2f65284b5ce8a4d81b1)
