test_ipagetkeytab: allow testing LDAP connection beyond bind operation
    
    Convert use_keytab() function into a context manager to allow additional
    operations to be done as part of the test. Also pass proper credentials
    cache file to the backend while connecting to LDAP so that right creds
    are in use.
    
    This is required to perform actual tests for use of the retrieved keys.
    
    Related: https://pagure.io/freeipa/issue/7953
    Reviewed-By: Christian Heimes <cheimes@redhat.com>