As long as your test suite just finds and runs all files in a given folder (without needing to explicitly "enable" them in some index file), this should work:

- create a NEW test file in `some/path/to/test.ext` (and back it up outside repo just in case)

- do NOT commit it in the repo

- `git bisect`

That way, bisect would check out different commits, but without touching `some/path/to/test.ext` because it's not tracked by git.

It could be also helpful to make git not see some file changes through the diff/status:

`git update-index --assume-unchanged path/to/some/file`

to trick git into thinking the file didn't change. (Although when you checkout a commit which did modify that file, this would crash).