How might we mark a test suite isn't parallalizable?

In HARNESS_OPTIONS we can set -jN to note we want parallel tests
running, but how can a particular module, which might be buried in the
dependency chain, tell the harness it can't do that?

It seems to me that by the time the tests are running, it's too late
because they are already in parallel and the best we can do is issue a
warning or decide to fail.