[th/opt-out-sphinx-test] nmci/tests: allow opt-out of test_nmci_doc() test via "NMCI_NO_SPHINX=1"
The result depends on the version of sphinx. It also requires a module from pip. That makes it cumbersome to run on a developer machine.
Allow to opt out from this check.
Similar to NMCI_NO_BLACK
, which skips test_black_code_fromatting()
.