ipatests: add missing tests for test_external_ca.py
    
    Some tests were missing from nightly definition:
    test_external_ca.py::TestExternalCAdirsrvStop
    test_external_ca.py::TestExternalCAInvalidCert
    test_external_ca.py::TestMultipleExternalCA
    
    Related to https://pagure.io/freeipa/issue/7743
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>