RFR: 8364203: ignore native mac tests on headless

[Johan Vos]

Add PlatformUtil.isHeadless() method.
Use that in tests to conditionally skip tests.

Fix JDK-8364203

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

Commit messages:
 - Add PlatformUtil.isHeadless() method.

Changes: https://git.openjdk.org/jfx/pull/1859/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1859&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364203
  Stats: 11 lines in 2 files changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1859.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1859/head:pull/1859

PR: https://git.openjdk.org/jfx/pull/1859