Switch from google-test to google-mock.

This is a backport to ```master``` of what @AlCutter did in 51a4cde3e0fb990f099c476e0c6f060fa8a506f6. Normally, I'd say [YAGNI](http://en.wikipedia.org/wiki/You_aren't_gonna_need_it), but I think @northshoredave might have some use for this in his fork, and the work is already done, so might as well minimize the pain...

See also: https://codereview.appspot.com/155620043/