This means that:
1. In test cases where minimal is the default (swift 5 without
-warn-concurrency), I added RUN lines for targeted, complete, and complete +
sns.
2. In test cases where complete is the default (swift 6, -warn-concurrency,
specified complete with -strict-concurrency), I added a send non-sendable run
line.
In each of these cases, I added additional expected-* lines as appropriate so
the tests can compile in each mode successfully.
Change the order of paths provided for `DYLD_LIBRARY_PATH` when running
tests for the back-deployed concurrency library, so they supersede the
ones in `/usr/lib/swift`. This way, we can test the back-deployed
concurrency library even on OS's that have the concurrency library in
them. It's not a complete test, because the standard library and
runtime will still vary, but can reproduce many failures more easily.
When using the back-deployed concurrency library, set the lit feature
`back_deploy_concurrency`. Update some tests that are unsupported with
back-deployment to check this.
Finally, add some logging when the concurrency runtime cannot be
tested at all, to help with debugging CI in the future.
