Merge changes Ic18443d4,I9e9af999,I10e099fe,I00cf6ec9,Ia1ea4ba7 into main

* changes:
  snapuserd: Reduce size of COWs in tests.
  snapuserd: Add unit tests for ReadWorker::ReadAlignedSector.
  snapuesrd: Build snapuserd_test on host.
  snapuserd: Add an ITestHarness implementation for hosts.
  snapuserd: Don't specify a path to TemporaryFile.