RFR: 8374655: Update CONTRIBUTING.md to ask PR authors to enable GHA in their repo

[Kevin Rushforth]

Adds a note to the "Testing" item for PR Authors to make sure that GHA is enabled for their personal fork. There is already a similar recommendation for Reviewers to check that.

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Commit messages:
 - 8374655: Update CONTRIBUTING.md to ask PR authors to enable GHA in their repo

Changes: https://git.openjdk.org/jfx/pull/2028/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2028&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374655
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/2028.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2028/head:pull/2028

PR: https://git.openjdk.org/jfx/pull/2028