RFR: 1242: The /clean command for backports is ignored

[Erik Joelsson]

When dealing with backport pull requests, Skara tries to automatically determine if the backport is "clean". This automatic check is rather picky, so the user is supposed to be able to override this result with the command "/clean". Issuing the command works so far as the bot will react to it and add the "clean" label, but then it will ignore the label and still not consider the pull request clean.

This patch fixes this and updates the test to also verify that the pull request gets the "ready" label.

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Commit messages:
 - SKARA-1242

Changes: https://git.openjdk.java.net/skara/pull/1239/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1239&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1242
  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1239.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1239/head:pull/1239

PR: https://git.openjdk.java.net/skara/pull/1239