RFR: 8369306: Implement invokeAndWait and finishTerminating on headless platform [v4]

[Jose Pereda]

> This PR implements `invokeAndWait` and `finishTerminating` for the headless platform and includes two system tests, one for each, that fail before this PR, and pass after it.

Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:

  Process feedback

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1934/files
  - new: https://git.openjdk.org/jfx/pull/1934/files/47a450a6..484f5cbd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1934&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1934&range=02-03

  Stats: 12 lines in 1 file changed: 10 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1934.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1934/head:pull/1934

PR: https://git.openjdk.org/jfx/pull/1934