RFR: 8249543: Force DirectBufferAllocTest to run with -ExplicitGCInvokesConcurrent

[rkennke at redhat.com]

DirectBufferAllocTest is unreliable when running with
+ExplicitGCInvokesConcurrent, because allocating DBB spreads
System.gc() calls all over concurrent GC cycles. It becomes more
reliable when running with -ExplicitGCInvokesConcurrent. (Shenandoah
defaults to +ExplicitGCInvokesConcurrent, other GCs don't as far as I
know.)

Bug:
https://bugs.openjdk.java.net/browse/JDK-8249543
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8249543/webrev.00/

Ok?

Thanks,
Roman