Ignoring non test files to avoid TS processing

[cgimenes]

I don't know why, but neotest runs this method every time I open a file.

I'm using lazy to lazy-load neotest, but after it gets loaded, I see a very noticeable slowness whenever I open a file that wasn't in a buffer.

I'm pretty sure this is a neotest issue, but I couldn't figure it out where the problem is in their codebase, that's why I'm opening this PR here. Maybe it will help someone else too.

[olimorris]

Thanks for this. Might be worth raising this upstream too.