RFR: 8244297: memory leak test utility [v6]

[Florian Kirmaier]

> It's based on the discussion of my previous PR: https://github.com/openjdk/jfx/pull/71
> 
> I Added test utility class copied from JMemoryBuddy and used it to simplify 4 of the existing unit tests.
> 
> It's a direct copy of my project [JMemoryBuddy](https://github.com/Sandec/JMemoryBuddy) without any changes.
> I'm also using it in most of the projects I'm involved with and in my experience, the tests with this Library are very
> stable. I can't remember wrong test results. Sometimes the memory behaviour of some libraries itself is not stable but
> the tests with JMemoryBuddy are basically always correct.

Florian Kirmaier has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8244297
  Fixed unit-test

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/204/files
  - new: https://git.openjdk.java.net/jfx/pull/204/files/adc870cf..6a861e43

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=204&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=204&range=04-05

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/204.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/204/head:pull/204

PR: https://git.openjdk.java.net/jfx/pull/204