Dig into potential issue with tokio start_paused

[sunshowers]

In oxidecomputer/omicron#2654 we found that a test with tokio::test(start_paused = true) that consistently worked on Linux was flaky on illumos.

Note that start_paused = true runs tokio in a mode where timers are paused, and auto-advance. Since all this code uses Tokio, pausing and auto-advancing timers should work in theory -- there's no reason for any of these operations to be tied to system timers.

This could be any of the following:

• an omicron code difference between Linux and illumos (though the test doesn't use any illumos-specific features and is completely OS-independent -- it uses MGS talking to sp-sim).
• a scheduler difference on illumos compared to Linux
• a Tokio difference between illumos on Linux
• a bug in Tokio