RFR: 7903836: Add GitHub Actions test workflow

[Christian Stein]

Please review this test workflow for building ApiDiff on every pull request.

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Commit messages:
 - Add GitHub Actions test workflow

Changes: https://git.openjdk.org/apidiff/pull/9/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=9&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903836
  Stats: 30 lines in 1 file changed: 30 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/apidiff/pull/9.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/9/head:pull/9

PR: https://git.openjdk.org/apidiff/pull/9