release-25.1: sql: deflake TestDropDatabaseDeleteData

[blathers-crl]

Backport 1/1 commits from #140653 on behalf of @iskettaneh.

/cc @cockroachdb/release

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This commit deflakes the test by doing two things:

1. Delay the start of the test till after the zone config range feed has started already. This is used to avoid a race where we update system.zones before the full reconciliation of zone configs has started. The full reconciliation ignores dropped databases, and if we write to system.zones before that, our write might not be reconciled. By delaying the test until the zone config range feed starts, we ensure that the full reconciliation has finished, and the range feed is ready to stream our changes.

2. Relaxes the test expectation that only the first table gets purged first. This is because there there is a race between the splits and GC queue where range might get GCed before splitting. This is more likely to happen in this test since increase the GC queue scan rate.

Release note: None

Fixes: #138185

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Release justification:

[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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  policy.

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