RFR: 8244297: memory leak test utility

[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.

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Commit messages:
 - JDK-8244297

Changes: https://git.openjdk.java.net/jfx/pull/204/files
 Webrev: https://webrevs.openjdk.java.net/jfx/204/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244297
  Stats: 322 lines in 5 files changed: 225 ins; 69 del; 28 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