[3.0] Labels pull requests that touch the test suite

[albertlast]

Description

The repository has a Unit Testing label, but .github/labeler.yml had no rule for it, so the Pull Request Labeler job never applied it. A change under tests/ was labelled by whatever production file happened to sit next to it in the same pull request, or not at all.

Five open pull requests touch the suite and none of them carry the label:

PR	Test files / total	Labels it did get
#9577	1 / 2	Navigation
#9575	1 / 3	Boards, Topics
#9537	2 / 2	Package Manager, Proxy, Backward compatibility
#9347	16 / 22	Installer, Localization
#9345	8 / 14	Installer, Localization

The one merged pull request that does carry Unit Testing, #9511, had it applied by hand.

The rule covers the suite itself plus the two files that decide how it runs:

"Unit Testing":
  - changed-files:
    - any-glob-to-any-file: ['tests/**', 'phpunit.xml.dist', '.github/workflows/phpunit.yml']

Placed alphabetically between Topics and Upgrader, matching the ordering of the rest of the file.

Issues References (Fixes|Related|Closes)