release-23.1: distsql: improve columnar operator test harness for decimals

[blathers-crl]

Backport 1/1 commits from #117563 on behalf of @yuzefovich.

/cc @cockroachdb/release

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We recently merged a change to add decimals with different numbers of trailing zeroes in the "interesting datums" set, and it made some existing tests fail because they used direct string comparison for equality. This commit adjusts the test harness to be smarter for decimals.

Fixes: #117543.

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.
• Backports should not break backwards-compatibility.
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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 @DrewKimball and @rafiss)