Shenandoah: ThreadMXBean synchronizer tests crash with aggressive heuristics

Tests that failed are:

java/lang/management/ThreadMXBean/LockedSynchronizers.java
java/lang/management/ThreadMXBean/MyOwnSynchronizer.java
Frequency of failure is around 1/10.

Reason for the failure is the following assertion:

# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/shenandoahBarrierSet.inline.hpp:246
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/asmehra/data/ashu-mehra/jdk/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.inline.hpp:246), pid=1427500, tid=1427517
# Error: Shenandoah assert_not_in_cset failed; Object should not be in collection set

Referenced from:
  interior location: 0x00007fc020003280
  outside of Java heap
  0x00007fc020003280 points into unknown readable memory: 0x0000000000000000 | 00 00 00 00 00 00 00 00

Object:
  0x00000000d01b6840 - klass 0x0000000800215ed8 java.util.concurrent.locks.ReentrantLock$NonfairSync
    not allocated after mark start
    not after update watermark
        marked strong
    not marked weak
        in collection set
  mark: mark(is_neutral no_hash age=0)
  region: | 6|CS |BTE d0180000, d01bff00, d01c0000|TAMS d01bff00|UWM d01bff00|U 255K|T 255K|G 0B|S 0B|L 49040B|CP 0

Forwardee:
  (the object itself)