release-23.2: sql: fix flake in TestTxnContentionEventsTable

[blathers-crl]

Backport 1/1 commits from #118372 on behalf of @michae2.

/cc @cockroachdb/release

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In causeContention we deliberately hold a transaction open using pg_sleep to block an update statement. The timing we're trying to achieve is:

1. transaction insert
2. update starts and blocks
3. transaction held open using pg_sleep

We were using a WaitGroup to order (2) after (1), but there was no synchronization to ensure (3) came after (2).

This commit adds a retry loop that checks crdb_internal.cluster_queries to ensure (3) comes after (2).

Fixes: #118236

Release note: None

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Release justification: test-only fix for a flake.

[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.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
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  here).
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @mgartner and @michae2)

[michae2]

This is a test-only change. Merging.