This:
- tests: drop the now broken MockedKafkaConsumer test class (I'll go remove it
from swh-journal after that diff [2]).
- migrate the journal client test to pytest (it would not accept to inject
fixtures otherwise)
- fixes the ci [1]
[1] https://jenkins.softwareheritage.org/job/DSEA/job/tests/266/console
[2] It got dropped already in the indexer which was using it as well (D3144)