test: remove pytest-timeout

[liuverta]

Impact and Context

Effectively reverts #2787 and #2690.

There is some sort of bad interaction between pytest-timeout and pytest-xdist that results in tests crashing. For references, see:

• pytest-xdist interop: signal only works in main thread pytest-dev/pytest-timeout#8
• pytest-xdist breaks asyncio-based code expecting to be run in the main thread pytest-dev/pytest-xdist#620

We had added pytest-timeout to fail faster when cases hang indefinitely. We can still rely on the pipeline timeout defined on the Jenkinsfile.

Risks and Area of Effect

Low risk: Nothing is likely to break; it will just take longer for test pipelines to fail on indefinitely-hanging tests (we could tighten the pipeline timeout if we'd like, since xdist is running our suites faster).

Low AoE: Only affects how the client test suite is run (mostly in CI, from reading the above-linked GitHub issues)

Testing

Ran test pipelines, which did not encounter the aforementioned bad interaction.

How to Revert

Revert this PR.