Code review comment for lp:~vexo/bzr/fix-1123460

I went ahead and added a couple basic test cases for verify-signatures --verbose, as it was completely uncovered previously and that didn't seem good. The minimal existing tests for verify-signatures were also oddly lumped in with sign-my-commits, so I split everything off into a new file.

After looking at it a bit more, the UTF-8 issue never actually had anything to do with verify-signatures, and the issue was really with generating the log, so I added the test case for it there. It seems like there might be a cleaner way of writing the test, but actually signing the test commit didn't seem to play nice with the way TestCaseWithTransport sets up its repository.