RFR: 8296919: Make system tests that detect memory leaks more robust by using JMemoryBuddy utility

Mon May 1 15:00:53 UTC 2023

Verified these changes on all platforms and saw no abnormalities in test execution.

This change is based on a preliminary patch done by @aghaisas , with some minor changes and the addition of one Leak-related web test. EventListenerLeak test was not touched, as it is a separate manual test app instead of a JUnit test.

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Commit messages:
 - Use JMemoryBuddy in Web leak tests
 - Use JMemoryBuddy in tests currently using System.gc()

Changes: https://git.openjdk.org/jfx/pull/1121/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296919
  Stats: 179 lines in 9 files changed: 24 ins; 118 del; 37 mod
  Patch: https://git.openjdk.org/jfx/pull/1121.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1121/head:pull/1121

PR: https://git.openjdk.org/jfx/pull/1121