feature request: add support for opening/running corresponding test files

[0xch4z]

For instance if I'm in file foo.py, I'd like to run :TestOpenFile<cr> to open foo_test.py if it exists.

Or if I run :TestFile from foo.py, it should look for the corresponding foo_test.py file and run the tests.

I'm happy to pick up the work for this if there's interest.

[genzade]

this would be a great addition to this awesome project. vim-test already implements this, i think.