[jfx18] RFR: 8280275: JUnit5 tests using Assumptions API fail to compile in some cases

[Kevin Rushforth]

Fixed a test dependency issue in `build.gradle` that causes a compilation failure when running `gradle test` if the `Assumptions` API from JUnit5 is used. I added a test that fails to compile without the build fix and passes with the fix.

This was first discovered when testing the patch for PR #715, which fails to compile on my local macOS system. After applying this fix, I can compile and run the test from that PR.

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Commit messages:
 - 8280275: JUnit5 tests using Assumptions API fail to compile in some cases

Changes: https://git.openjdk.java.net/jfx/pull/720/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=720&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280275
  Stats: 6 lines in 2 files changed: 4 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/720.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/720/head:pull/720

PR: https://git.openjdk.java.net/jfx/pull/720