Subsequent `/test` invocations should cancel previous runs

[binarylogic]

This is a nice to have feature but is not necessary. In other words, I only want to implement this if it's less than 2 hours of work.

Calling /test should cancel previous runs to save on cost.

[MOZGIII]

This is not yet supported out of the box, afaik. Though, we can still come up with a custom workaround!
Refs:

• https://github.community/t5/GitHub-Actions/Github-actions-Cancel-redundant-builds/td-p/29549
• https://github.community/t5/GitHub-Actions/Is-it-possible-to-cancel-previous-jobs-automatically-when-the/td-p/37139