Enable parallelism in vmTestbase/gc/vector tests

Similar to JDK-8315774, we want to unblock parallelism for more `hotspot:tier4` tests. `vmTestbase/gc/vector` is a GC stress test suite. These look similar to what we run as GCBasher. So there seem to be no reason to run these tests exclusively.

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.

Current run takes about 7 minutes:

CONF=linux-x86_64-server-fastdebug make test TEST=vmTestbase/gc/vector 2762.06s user 78.94s system 672% cpu 7:02.25 total