release-23.2: randgen: avoid overflow in computed column expressions

[blathers-crl]

Backport 1/1 commits from #125946 on behalf of @rafiss.

/cc @cockroachdb/release

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The computed column definition could lead to overflow errors since it adds a number that is randomly chosen and could be very large. Using absolute value instead avoids this.

fixes #125747
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 

[annrpom]

lgtm