Fix cross-worker cache contamination in parallel test runs (#23106)

RQQueueTestMixin cleared RQ queues with FLUSHALL, which empties every
database on the Redis server — including the caching database, whose
'config'/'config_version' keys are shared by all parallel test workers.
A flush landing mid-test forces an unrelated worker to re-read
core_configrevision, adding two queries to the affected request. Flush
only the queue's own database instead.

Also stop GraphQLDeferredColumnTestCase from comparing total query counts
between two requests, which is what surfaced the race as intermittent
"Query count grew from 7 to 9" failures in CI. Assert that the target
table is read exactly once per request instead, as #23034 did for the
equivalent custom fields test.