RFR: 8361827: [TESTBUG] serviceability/HeapDump/UnmountedVThreadNativeMethodAtTop.java throws OutOfMemoryError

[Richard Reingruber]

This pr makes sure to call System.gc() before each test in UnmountedVThreadNativeMethodAtTop.java

The intention is to get rid of unreachable objects representing loaded heap dumps from previous tests.

This prevents OutOfMemoryErrors. Parsing the smaller dumps is faster too.

The PR also includes a reproducer for convenient testing of the pr.

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Commit messages:
 - Revert "Reproduce OOM with duplicate test case"
 - Fix issue: System.gc() before each test
 - Reproduce OOM with duplicate test case

Changes: https://git.openjdk.org/jdk/pull/26278/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26278&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361827
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26278.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26278/head:pull/26278

PR: https://git.openjdk.org/jdk/pull/26278