RFR: 8364203: Ignore native mac tests on headless [v2]

[Johan Vos]

> Add PlatformUtil.isHeadless() method.
> Use that in tests to conditionally skip tests.
> 
> Fix JDK-8364203

Johan Vos has updated the pull request incrementally with one additional commit since the last revision:

  Exclude setup/teardown in case of headless platform

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1859/files
  - new: https://git.openjdk.org/jfx/pull/1859/files/ceec1c3e..0a7a7be9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1859&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1859&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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