RFR: 1990: Fix MailingListNotifierTests#testMailingListBranch

[Zhao Song]

Due to [SKARA-1979](https://bugs.openjdk.org/browse/SKARA-1979), when generating a notification email for the first branch of a repository, all refs except pr/X branches are regarded as potential candidates. 

Therefore, this test sometimes fails because the master branch and testlock branch are identical, and it's uncertain which one will be selected as the base for the new branch.

-------------

Commit messages:
 - SKARA-1990

Changes: https://git.openjdk.org/skara/pull/1543/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1543&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1990
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1543.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1543/head:pull/1543

PR: https://git.openjdk.org/skara/pull/1543