Improve proxy connection tests.

Speed up some tests by removing unnecessary async calls.
Check errors in onSuccess callbacks are handled.
Check that requestDevices worker is set up, and that it does not propagate state change IDLE -> IDLE beyond the UI.

Bug: 319302757
Test: npm run test:unit:ci
Change-Id: Ic6c0874ede41863d8d82ed5142ba222df590c1f0