tests: only flush() the kafka journal writer once per test
instead of flushing it n times per test. Since the call to kafka Producer.flush() takes about 1s, reducing the number of calls to this method significantly reduce the execution time of the tests.
This required a small refactoring of the JournalBackfiller class to make the journal writer live out of the scope of the run() so the test can access the journal writer instance and call the flush() method.
Requires swh-journal >= 1.2.0 (see swh-journal!229 (closed)).
Migrated from D8740 (view on Phabricator)