RFR: 7903857: apidiff: Include test run in GitHub Actions workflow

[Christian Stein]

Please review this change to actually run ApiDiff's self-tests on all three major platforms.

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Commit messages:
 - CODETOOLS-7903857: apidiff: Include test run in GitHub Actions workflow

Changes: https://git.openjdk.org/apidiff/pull/23/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=23&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903857
  Stats: 14 lines in 1 file changed: 8 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/apidiff/pull/23.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/23/head:pull/23

PR: https://git.openjdk.org/apidiff/pull/23