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 <module> from libnmstate.validator import validate_boolean
  E   ImportError: cannot import name 'validate_boolean'
  !!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
2 jobs for !861 with lr/bad-nmstate in 1 minute and 55 seconds
detached
Status Name Job ID Coverage
  Test
manual TestResults #14944952
manual
passed UnitTests #14944951

00:01:55