RFR: 7903857: apidiff: Include test run in GitHub Actions workflow [v4]

[Christian Stein]

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

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Try other MSYS2 installation path

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Changes:
  - all: https://git.openjdk.org/apidiff/pull/23/files
  - new: https://git.openjdk.org/apidiff/pull/23/files/50ba912a..db262374

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=apidiff&pr=23&range=03
 - incr: https://webrevs.openjdk.org/?repo=apidiff&pr=23&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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