Log unit test improvements

The log unit tests were often failing when run in parallel, which
was caused by incorrect dependencies between tests.  Some tests
were not cleaning up after themselves properly, and the changes
they made were influencing other tests.  This commit makes all of
the tests independent of each other and ensures that proper
cleanup is being done after the tests that alter global log state.

Change-Id: I9f8bb4096963077737e556d2808791123cec441b
Closes-Bug: #1275890
Related-Bug: #1272035
Clear time override in test_periodic

The periodic tasks tests were not clearing their time override
properly, so it was persisting into the timeutils test cases when
they were run first.

Change-Id: Idaa2bddba4943f5f8776a883228c56d8503097e7
Closes-Bug: #1275152
Related-Bug: 1272035
Merge "Log unit test improvements"
Merge "Clear time override in test_periodic"