release-24.2: importer: skip TestImportComputed under race

[blathers-crl]

Backport 1/1 commits from #135962 on behalf of @yuzefovich.

/cc @cockroachdb/release

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We just saw a test failure where this test failed seemingly due to overloaded cluster (3 node, with external process multi-tenancy). I think we should skip it under race like we do a few other tests in this file already.

Fixes: #135894.

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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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 @rytaft)