RFR: 8336882: Convert CssStyleHelperTest to use JUnit 5

[John Hendrikx]

Conversion of the CssStyleHelperTest to JUnit 5 for PR's #1503 and #1505

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Commit messages:
 - Convert CssStyleHelperTest to JUnit 5 test

Changes: https://git.openjdk.org/jfx/pull/1515/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1515&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336882
  Stats: 9 lines in 1 file changed: 0 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1515.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1515/head:pull/1515

PR: https://git.openjdk.org/jfx/pull/1515