Add ipa-getkeytab and ipa-rmkeytab CLI test cases for user principals to ipatests/test_cmdline/test_ipagetkeytab.py to improve upstream test coverage and enable earlier detection of failures in upstream CI.

The existing test coverage in test_ipagetkeytab.py focuses on service principals. These new test cases extend coverage to user principals, exercising the following areas:

Quiet mode, access control, and credential cache handling
--server / -s with valid and invalid hostnames
--principal / -p with unknown, invalid-realm, bare, and realm-qualified user principals
--keytab / -k creation, enctype content, and invalid file paths
-e single enctype filtering with kinit validation
--password / -P key rotation
-D / -w bind DN error cases (empty DN, wrong password, missing password)
ipa-rmkeytab removal by principal, by realm, and invalid keytab path handling

Test cases that duplicate existing upstream coverage are skipped with comments referencing the original test locations.