RFR: tester: allow configurable allowlist

[Erik Helin]

Hi all,

please review that patch that enables a configurable allowlist for the tester bot. I also added a unit test for the new
functionality.

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

Thanks,
Erik

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Commit messages:
 - test-bot: allow configurable allowlist

Changes: https://git.openjdk.java.net/skara/pull/753/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=753&range=00
  Stats: 125 lines in 7 files changed: 96 ins; 0 del; 29 mod
  Patch: https://git.openjdk.java.net/skara/pull/753.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/753/head:pull/753

PR: https://git.openjdk.java.net/skara/pull/753