RFR[JDK11]: 8204691: HeapRegion.apply_to_marked_objects_other_vm_test fails with assert(!hr->is_free() || hr->is_empty()) failed: Free region 0 is not empty for set Free list #

[Kim Barrett]

Please review this fix of the HeapRegion gtest.

The test modifies a region's "top" to unexpected values without
ensuring that no allocation might use the region and no GC might run
while the region is in that invalid state.  We solve this by executing
the test code in its very own safepoint, and by saving and then
restoring the region's top back to its original value before completing
the test.  And since we are doing all that, there's no longer any
reason to run the test in a separate VM.

CR:
https://bugs.openjdk.java.net/browse/JDK-8204691

Webrev:
http://cr.openjdk.java.net/~kbarrett/8204691/open.00/

Testing:
mach5 tier1 (where gtests are run).

I wasn't able to reproduce the failure, but the issues these changes
address can account for the one failure that's been reported.