refactor(testutil): remove unused api

[julienrbrt]

Description

These APIs aren't currently maintained and produce flaky tests:
As these never were released, we should imho remove them from v050 (like done for #16864).
imho, we should not release something incomplete that may make our testing story less clear.

If preferred, I can remove them from main and backport to v0.50 if the work on this won't resume.

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