RFR: 7903857: Include test run in GitHub Actions workflow

[Christian Stein]

Please review this change to run APIDiff's self-tests for each pull request.

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

Changes: https://git.openjdk.org/apidiff/pull/25/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=25&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903857
  Stats: 14 lines in 1 file changed: 4 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/apidiff/pull/25.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/25/head:pull/25

PR: https://git.openjdk.org/apidiff/pull/25