RFR: 2233: TestHost does not set assignees correctly for backports

[Erik Duveblad]

Hi all,

please review this small patch that makes `TestHost` set assignees correctly for any backport it creates. For now have I opted to just throw if the `HostUser` isn't found. In the future we could instead invoke the `RestRequest.ErrorTransform` if we ever have a need to test what happens when the value for the `assignee` property in the body to the backports endpoint contains an unknown username (we currently do not have that need).

Thanks,
Erik

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Commit messages:
 - skara-2233

Changes: https://git.openjdk.org/skara/pull/1636/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1636&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2233
  Stats: 7 lines in 1 file changed: 4 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1636.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1636/head:pull/1636

PR: https://git.openjdk.org/skara/pull/1636