nmci/test: print output of `black` in case of test failure

pytest nicely captures and shows the output. We now print
the `black` command line and the diff that `black` generated.
Status Name Job ID Coverage
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/1470/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-test-upstream/1470/

00:17:59