RFR: 8244297: Provide utility for testing for memory leaks [v7]

[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
  Updated JMemoryBuddy based on codereview.

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/204/files
  - new: https://git.openjdk.java.net/jfx/pull/204/files/6a861e43..427170b8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=204&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=204&range=05-06

  Stats: 36 lines in 1 file changed: 18 ins; 0 del; 18 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