RFR: 8264411: serviceability/jvmti/HeapMonitor tests intermittently fail due to large TLAB size

[Hui Shi]

…ue to large TLAB size

serviceability/jvmti/HeapMonitor tests intermittently fail when using PS/Serial GC, original test has implicit assumptions on TLAB size and depends on allocate fix amount of objects to consume TLAB and trigger object sampling. These tests will fail if TLAB is above 20M (this can easily happen when using PS/Serial GC and heap is large), when allocation can not consume current TLAB and _byte_until_sample.

Fix in tests is adding an explicit GC to consume current TLAB.
Running on 256G memory machine, make run-test CONF=release TEST="test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/" 'JTREG=JOBS=12;VM_OPTIONS=-XX:ActiveProcessorCount=1'

before fix: 6 or 7 tests in 20 tests intermittently fail
after fix: no failure in 100 runs release/fastdebug

This might also fix https://bugs.openjdk.java.net/browse/JDK-8225313

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Commit messages:
 - 8264411: serviceability/jvmti/HeapMonitor tests intermittently fail due to large TLAB size

Changes: https://git.openjdk.java.net/jdk/pull/3265/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3265&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264411
  Stats: 22 lines in 2 files changed: 17 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3265.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3265/head:pull/3265

PR: https://git.openjdk.java.net/jdk/pull/3265