Enable parallelism in vmTestbase/gc/gctests/LargeObjects tests

We want to unblock parallelism for more `hotspot:tier4` tests. `vmTestbase/gc/gctests/LargeObjects` is a small test suite, running a series of effectively single-threaded tests. Note that the tests themselves can be slow, see JDK-8307788, which gets even worse when they are not parallel. There seem to be no reason to run these tests exclusively.

Current `vmTestbase/gc/gctests/LargeObjects` passes in 23 minutes with only 1.5 CPU taken:
CONF=linux-x86_64-server-fastdebug make images test 2068.61s user 39.33s system 151% cpu 23:11.72 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.