Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
* Enable running tests from test/Concurrency/ directory in freestanding/minimal presets
* Mark failing Concurrency tests as XFAIL/SKIP on freestanding/minimal
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.
