Add instructions to avoid unwanted tests for bug-fix-agent

[suhaibmujahid]

Might want to add instructions to avoid "stupid" tests that Claude often generates, @padenot do you have any tips from your usage on this?

Originally posted by @marco-c in #6001 (comment)

[padenot]

My instructions are significantly more verbose, and specific to my needs, mimicking what a Gecko developer would do. I reckon this is best done as a sub-file in the skill that will guide the agent to determine what test is best in what scenario.

Can you assign me to this issue, so I can do a "first cut", and we can iterate?