sql: fix race in TestRaceWithBackfill
sql: fix race in TestRaceWithBackfill

This test was broken when we moved it to use TestCluster.
With TestCluster along with distsql based backfills,
multiple backfills can be running on different servers
at the same time.

fixes #13622

paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach