RFR: 8356687: [TestBug] SWT tests do not run on macOS

[Jayathirth D V]

We have enabled swt tests under [JDK-8169285](https://bugs.openjdk.org/browse/JDK-8169285) but they were disabled on macOS as they throw below exception :


FXCanvasScaledTest STANDARD_OUT
    ***WARNING: Display must be created on main thread due to Cocoa restrictions. Use vmarg -XstartOnFirstThread

FXCanvasScaledTest > initializationError FAILED
    org.eclipse.swt.SWTException: Invalid thread access


Using `-XstartOnFirstThread` flag in gradle was not reliable and the issue was tracked under https://issues.gradle.org/browse/GRADLE-3290 and later under https://github.com/gradle/gradle/issues/864. This gradle issue is resolved and we can use `-XstartOnFirstThread` and enable SWT tests on macOS.

With `-XstartOnFirstThread`, SWT tests run fine on macOS.

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Commit messages:
 - 8356687: [TestBug] SWT tests do not run on macOS

Changes: https://git.openjdk.org/jfx/pull/1888/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1888&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356687
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1888.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1888/head:pull/1888

PR: https://git.openjdk.org/jfx/pull/1888