RFR: Add test subcommand to git-pr

[Erik Helin]

Hi all,

please review this small pull request that adds the `test` subcommand to `git pr`, so that one can now write `git pr test <PR-ID>`.

Thanks,
Erik

## Testing
- [x] Manual testing of `git pr test`

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Commits:
 - 4e070615: Add test subcommand to git-pr

Changes: https://git.openjdk.java.net/skara/pull/282/files
 Webrev: https://webrevs.openjdk.java.net/skara/282/webrev.00
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/282.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/282/head:pull/282

PR: https://git.openjdk.java.net/skara/pull/282