RFR: 8296919: Make system tests that detect memory leaks more robust by using JMemoryBuddy utility [v3]

[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:

  Minor review updates
  
  * Changed ArrayList<> to Collection<> in one of assertCollectable-s
  * Updated LeakTest to use new assertCollectable helpers

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1121/files
  - new: https://git.openjdk.org/jfx/pull/1121/files/158ed40a..0b8b058f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1121&range=01-02

  Stats: 13 lines in 2 files changed: 0 ins; 8 del; 5 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