Duplicate select Swift Concurrency tests for TSan
I am putting these tests into a new `Sanitizers/tsan` subfolder so we
can keep the same test file name (avoid need for `tsan-` prefix). This
should increase the chance that changes to the tests make it to the TSan
duplicate.
I will move and rename existing sanitizer tests (including ASan) tests
to keep consistency in a follow-up PR.
