RFR: 8310666: gradle validateSourceSets task not run when TEST_ONLY=true

[Ambarish Rapte]

The validateSourceSets task are created only for the test tasks. [check source](https://github.com/openjdk/jfx/blob/eb7de72dafecbedc83c2215b6aed7432d4ec80f9/build.gradle#L1848)
So, validateSourceSets task must be executed when running test tasks even when TEST_ONLY=true.
This had caused a regression [JDK-8310654](https://bugs.openjdk.org/browse/JDK-8310654), where we missed a failing scenario as validateSourceSets did not run with -PTEST_ONLY=true

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

Commit messages:
 - Don't skip validateSourceSets task when TEST_ONLY=true

Changes: https://git.openjdk.org/jfx/pull/1237/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1237&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310666
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1237.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1237/head:pull/1237

PR: https://git.openjdk.org/jfx/pull/1237