release-23.2: sql/logictest: fix flakes in select_for_update_read_committed

[blathers-crl]

Backport 1/1 commits from #112701 on behalf of @mgartner.

/cc @cockroachdb/release

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The select_for_update_read_committed tests were flaking because not
all statements were being run under READ COMMITTED isolation. The logic
test infrastructure does not allow fine-grained control of sessions, and
setting the isolation level in one statement would only apply to a
single session. Subsequent statements are not guaranteed to run in the
same session because they could run in any session in the connection
pool. This commit wraps each statement in an explicitly transaction with
an explicit isolation level to ensure READ COMMITTED is used.

In the future, we should investigate allowing fine-grained and explicit
control of sessions in logic tests.

Fixes #112677

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:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[michae2]

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