Add testing instructions to CONTRIBUTING_HOWTO.txt

[yonic2]

Following the discussion with @ffilz, there are a few tests that are being run manually by the maintainers before merging a branch (e.g. pynfs).
It would be nice to have those documented in CONTRIBUTING_HOWTO.txt so the branch authors can reproduce the errors and fix them.

[xiaods]

@yonic2 contribute is welcome