Enable parallelism in vmTestbase/nsk/stress/stack tests

Similar to JDK-8315937, we want to unblock parallelism for more `hotspot:tier4` tests. `vmTestbase/nsk/stress/stack` is a small, quick test suite. There seem to be no reason to run these tests exclusively. They currently complete in about 3 minutes:
 639.00s user 12.82s system 348% cpu 3:07.29 total

We should consider enabling parallelism for them and get improved test performance. Currently it is blocked by `TEST.properties` with `exclusiveAccess.dirs` directives in them.