RFR: 8296919: Make system tests that detect memory leaks more robust by using JMemoryBuddy utility [v5]

[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 two additional commits since the last revision:

 - JMemoryBuddy: Add javadoc comments
 - Remove thread sleeps before JMemoryBuddy calls
   
   JMemoryBuddy performs sleeps to check if objects are collectable by
   itself, there is no reason to do those.

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1121/files
  - new: https://git.openjdk.org/jfx/pull/1121/files/5ea23cf9..492d670d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=03-04

  Stats: 18 lines in 3 files changed: 13 ins; 5 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