general/nmstate: run nmstate tests against the same git tree

Otherwise it would attempt to use whatevers happens to be shipped in the package installed, possibly of mismatching version, resulting in no end of mayhem:

tests/lib/validator_test.py:23: in from libnmstate.validator import validate_boolean E ImportError: cannot import name 'validate_boolean' !!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!