release-22.2: importer: only check import *atomicity* in TestImportWorkerFailure

[yuzefovich]

Backport 1/1 commits from #108626.

/cc @cockroachdb/release

---

Five years ago, in #26881, we changed import to retry on worker failures, which made imports much more resilient to transient failures like nodes going down. As part of this work we created TestImportWorkerFailure which shuts down one node during an import, and checks that the import succeeded. Unfortunately, this test was checked-in skipped, because though imports were much more resilient to node failures, they were not completely resilient in every possible scenario, making the test flakey.

Two months ago, in #105712, we unskipped this test and discovered that in some cases the import statement succeeded but only imported a partial dataset. This non-atomicity seems like a bigger issue than whether the import is able to succeed in every possible transient failure scenario, and is tracked separately in #108547.

This PR changes TestImportWorkerFailure to remove successful import as a necessary condition for test success. Instead, the test now only checks whether the import was atomic; that is, whether a successful import imported all data or a failed import imported none. This is more in line with what we can guarantee about imports today.

Fixes: #102839

Release note: None

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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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[michae2]

Did we see a failure on 22.2?

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

[yuzefovich]

Yes, on 22.2.17-rc branch #113844.