RFR: 8248551: [TestBug] Ignore two failing FXML unit tests which use Nashorn script engine

[Ajit Ghaisas]

Issue : https://bugs.openjdk.java.net/browse/JDK-8248551
Fix : Added a check whether 'Nashorn' script engine is available before running the identified tests, otherwise tests
are ignored.

Testing :
1) With JDK-14, these tests pass- but they log a warning (same as existing behavior without this PR)
2) With JDK-15-ea, these tests get ignored.

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

Commit messages:
 - check for Nashorn engine

Changes: https://git.openjdk.java.net/jfx/pull/259/files
 Webrev: https://webrevs.openjdk.java.net/jfx/259/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248551
  Stats: 19 lines in 1 file changed: 18 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/259.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/259/head:pull/259

PR: https://git.openjdk.java.net/jfx/pull/259