CQLSH completion test broken by #12236

The commit of CASSANDRA-12236 appears to have broken cqlsh completion tests. For the error message I suspect this may have to do with something like the test comparing the completion output to what DESCRIBE shows and the later now doesn't include the cdc option by default.

Anyway, I'm not really familiar with cqlsh completion nor it's test so I'm not sure what's the best option. I don't think we want to remove cdc from completion so I suspect we want to either special case the test somehow (no clue how to do that), or make the test run with cdc enabled so it doesn't complain (which I think mostly apply a change to the CI environment since it seems the tests themselves don't spin up the cluster).

Anyway, pushing that fix to someone else as I'm not competent here and I have't even be able to run those cqlsh test so far (getting stuck at the test telling me that "No appropriate python interpreter found", even though I totally have an appropriate interpreter and cqlsh works perfectly if I execute it directly).