RFR: 8296919: Make system tests that detect memory leaks more robust by using JMemoryBuddy utility [v6]

[Lukasz Kostyra]

> 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.

Lukasz Kostyra has updated the pull request incrementally with one additional commit since the last revision:

  Add an EDT no-op and sleep for a bit
  
  On some platforms Swing memory leak tests occasionally failed, this
  might help to mitigate this problem.

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1121/files
  - new: https://git.openjdk.org/jfx/pull/1121/files/492d670d..1230c8b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=04-05

  Stats: 10 lines in 2 files changed: 10 ins; 0 del; 0 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