chore(ci): test dependencies should use latest

[dougch]

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Problem:

The CI broke when two testing dependencies updated their MSRV to something newer.
We addressed it by pinning to older versions in #2488.

Solution:

Pinning to older versions is a short term workaround; in general we need to limit our MSRV testing to unit tests and let other test dependencies use latest to avoid needless toil.

Requirements / Acceptance Criteria:

Out of scope: