release-23.2: sqlccl: de-flake TestActiveCancelSession

[blathers-crl]

Backport 1/1 commits from #124564 on behalf of @yuzefovich.

/cc @cockroachdb/release

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This commit fixes a possible race in TestActiveCancelSession. In particular, that test uses two connections in the following manner:

• conn1 runs pg_sleep query that is intended to run forever
• conn2 executes CANCEL SESSION for conn1.

Since pg_sleep query is executed in a separate goroutine, previously it was possible for CANCEL SESSION statement to be issued before either the separate goroutine was spun up or it began executing the pg_sleep query, and in both cases this would result in connection reset by peer error which is not what the test expects. This race is now fixed by blocking conn2 until it sees the pg_sleep query in the SHOW CLUSTER QUERIES output.

Fixes: #123639.

Release note: None

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Release justification: test-only change.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[cockroach-teamcity]

This change is