testdrive: unflake introspection-sources.td

[teskje]

The queries testing that all workers report the same dataflows, operators, channels, etc. would get confused by transient dataflows created for oneshot selects. These dataflows get created and dropped very quickly, so it is likely for different workers to have different parts of them installed at any given time. To unflake the queries we need to filter out transient dataflows.

Motivation

• This PR fixes a recognized bug.

Fixes https://github.com/MaterializeInc/database-issues/issues/9802

Tips for reviewer

As is customary, I wasn't able to reproduce the issue locally, but it makes sense that it would occur under different timings.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• 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).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[def-]

Thanks Jan!

[teskje]

TFTR!