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.
The 5.5/5.6 concurrency runtime in the concurrency backdeploy library
just doesn't have the pieces necessary to emit the desired error
message. Disabling it in that environment.
Commit afc5116ef0 added the support to track re-used continuations.
* Enable running tests from test/Concurrency/ directory in freestanding/minimal presets
* Mark failing Concurrency tests as XFAIL/SKIP on freestanding/minimal
