RFR: 2198: Backport PRs should check if a CSR is required

[Zhao Song]

This patch is trying to let the pr bot automatically requires csr for a backport PR if one of the associated has a resolved CSR.

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Commit messages:
 - fix test
 - update
 - add test
 - update
 - SKARA-2198

Changes: https://git.openjdk.org/skara/pull/1623/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1623&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2198
  Stats: 206 lines in 3 files changed: 171 ins; 19 del; 16 mod
  Patch: https://git.openjdk.org/skara/pull/1623.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1623/head:pull/1623

PR: https://git.openjdk.org/skara/pull/1623