This warnings don't give much benefits for developers. Code completion
UI tends to show them unusable. But usually, developers can modify the
context to accept async calls, e.g. by wrapping it with `Task { }`
rdar://126737530
The Python build system always enables concurrency, but CMake has it
disable by default. Collaborators that do not use the Python build
system and use directly CMake will have it disable, unless they
explicitely enable it. If the tests are not marked as requiring the
concurrency features, the tests will fail to execute when concurrency is
disabled.
The changes add the `REQUIRES: concurrency` line to many tests that deal
with concurrency, but wasn't marked as such.
