Add branch to test result mail subject lines

Problem/Motivation

When I get email results for core HEAD test fails, they have subjects like:

Drupal core test fail!  on PHP 7.4 & SQLite 3.27

Unfortunately, neither the subjects nor the body of the email say which branch this test failure is for. I monitor test results for 3-5 different branches of core daily, so it would be really helpful and save committers a lot of time to be able to tell which branch the fail was for.

Proposed resolution

Put the core branch in the email subject.

The code should be somewhere near: https://git.drupalcode.org/project/project_issue_file_test/-/blob/7.x-3....

Remaining tasks

TBD

User interface changes

N/A

API changes

TBD

Data model changes

TBD