RFR: 7903160: Run self-tests for jtreg on MacOS and Windows

[Christian Stein]

CI builds including self-tests for `jtreg` are run on Linux.

Now, they also run on MacOS and Windows.

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Commit messages:
 - 7903160: Run self-tests for jtreg on MacOS and Windows

Changes: https://git.openjdk.java.net/jtreg/pull/75/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=75&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903160
  Stats: 10 lines in 1 file changed: 7 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/75.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/75/head:pull/75

PR: https://git.openjdk.java.net/jtreg/pull/75