test_smb: skip test_smb_service_s4u2self for fed31
The test test_integration/test_smb.py::TestSMB::test_smb_service_s4u2self
is expected to fail in Fedora <= 31 as it requires krb >= 1.18
that is shipped from fedora 32 only.
Skip the test depending on the fedora version.
Fixes: https://pagure.io/freeipa/issue/8505
Reviewed-By: Rob Crittenden <rcritten@redhat.com>