storage: Fix testdrive test to not overwrite schema

[moulimukherjee]

This is correcting a testdrive test which inadvertently worked.

So, apparently it's possible to create multiple sinks on the same kafka topic and we just publish a new avro schema? I guess we missed the test was using the same topic to publish different schema and the overwrite wasn't taking long. With some refactoring in #21852, the CI started continuously failing https://buildkite.com/materialize/tests/builds/64722#018add21-f1a5-4258-bf3a-01148d0e33a6.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: