RFR: 1252: Improve the output message about the command `csr`

[Guoxiong Li]

Hi all,

Currently, when a user, who is not the author of the pr and is not a reviewer, uses the command `/csr unneeded`, the bot will output the message `only the pull request author and [Reviewers](https://openjdk.java.net/bylaws#reviewer) are allowed to require a CSR.`. The message is good when meeting the command `/csr` or `/csr needed` but it is not good when meeting the command `/csr unneeded` or `/csr uneeded`.

This patch changes the message `are allowed to require a CSR` to `are allowed to use the csr command` and adds the corresponding test cases.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - 1252: Improve the output message about the command `csr`

Changes: https://git.openjdk.java.net/skara/pull/1243/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1243&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1252
  Stats: 28 lines in 2 files changed: 24 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/1243.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1243/head:pull/1243

PR: https://git.openjdk.java.net/skara/pull/1243