Merge changes Ib33dc593,I6550682f,Ic0681cbf,Iaaf96a37,I89b15492 into main

* changes:
  snapuserd: Factor a base class out of tests.
  snapuserd: Add error propagation to tests.
  snapuserd: Add diagnostics for debugging races.
  snapuserd: Fix race condition in HandleManager shutdown.
  snapuserd: Remove test dependence on LoopDevice.