RFR: 8234110: SwingFXUtilsTest is unsuitable for unit test framework

[Kevin Rushforth]

This fixes [JDK-8234110](https://bugs.openjdk.java.net/browse/JDK-8234110) by moving `SwingFXUtilsTest` from the `:swing` project to the `:systemTests` project. As explained in the JBS issue, `SwingFXUtilsTest` must be run in its own VM, since it starts and stops the FX runtime, so must be in the system tests project.

I note that I could have instead modified `build.gradle` to specify `forkEvery = 1` in the `test` task of the `:swing` project, but since other similar tests are already in `systemTests` this seems cleaner.

There are no actual changes needed to any of the moved files, since the package name remains the same.

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Commits:
 - 141a388a: 8234110: SwingFXUtilsTest is unsuitable for unit test framework

Changes: https://git.openjdk.java.net/jfx/pull/35/files
 Webrev: https://webrevs.openjdk.java.net/jfx/35/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234110
  Stats: 0 lines in 6 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/35.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/35/head:pull/35

PR: https://git.openjdk.java.net/jfx/pull/35