RFR: jcheck: conditionally check backport reviewers

[Erik Helin]

Hi all,

please review this patch that makes it possible to configure whether reviewers are required or not for backport
commits. The `reviewers` check now supports the configuration `backports=check` or `backports=ignore` with
`backports=ignore` being the default. We will still check for invalid reviewers and self-reviews, it is just the
"number of reviewers" that can be disabled for backport commits.

Testing:
- [x] Added three new unit tests
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - jcheck: conditionally check backport reviewers

Changes: https://git.openjdk.java.net/skara/pull/884/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=884&range=00
  Stats: 73 lines in 3 files changed: 59 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/884.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/884/head:pull/884

PR: https://git.openjdk.java.net/skara/pull/884