snapuserd: Add an ITestHarness implementation for hosts.

This adds an implementation of ITestHarness suitable for running tests
on the host. IUserDevice and ProcessRequests are implemented just enough
for shutdown of ReadWorker to work. Most snapuserd tests are focused on
merging, and do not need a ReadWorker accepting requests.

Bug: 288273605
Test: snapuserd_test
Change-Id: I00cf6ec941fb6423290f7a299e5321adea7d8919