tests: find a reliable strategy for testing system and managed interpreters

[lucab]

Followup from #7451.

As noted as a comment there in #7451 (comment), some tests may require switching between managed and system interpreters.
While this works fairly well in CI, there is some implicit nondeterminism related to system setup and network fetches. This makes such tests brittle and unfriendly for other developers and packagers.