Unit tests for misspelled policy names.

Unit tests for misspelled policy names.

This verifies that we detect and report misspelled policy names
regardless of whether "RemoteAccessHost" appears as a prefix,
suffix or a substring.  I felt adding this test is important
to "document" the behavior asked about in
https://codereview.chromium.org/966433002/diff/120001/remoting/host/policy_watcher.cc#newcode109

The new test verifies expected behavior via MockLog class moved to base/test in
crrev.com/966423003.  I feel that the benefit of documenting and automatically
verifying the expected behavior outweights the small maintenence cost of an
additional test and the slight ickyness of verifying the behavior by looking at
log output.

BUG=

Review URL: https://codereview.chromium.org/979823004

Cr-Commit-Position: refs/heads/master@{#319172}