StringInternCleanup test times out

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/tests-stringintern/webrev.01/

Fun fact: recent improvements to Pacer make workloads more resistant to allocation failures, and
making ClassUnloadingWithConcurrentMark off by default made StringInternCleanup time out. Because it
waits for degen/full gc to purge the StringTable now, and it eventually succeeds after the
allocation failure, but it now takes a while. It takes more time with precise pacer.

The way out it to make the test 5x shorter and selectively enable ClassUnloadingWithConcurrentMark.

Testing: StringInternCleanup

Thanks,
-Aleksey