RFR: 1990: Fix MailingListNotifierTests#testMailingListBranch [v2]

[Zhao Song]

On Mon, 14 Aug 2023 16:59:23 GMT, Zhao Song <zsong at openjdk.org> wrote:

>> 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.
>
> Zhao Song has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - update
>  - update

Thanks for the review!

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PR Comment: https://git.openjdk.org/skara/pull/1543#issuecomment-1677740351