trivial/Timeit: Add a helper for timing tests
trivial/Timeit: Add a helper for timing tests

While we have '-cpuprofile' available, it doesn't
give useful information when that time is spent
inside time.Sleep(). (We don't spend any CPU time
while sleeping. :)

Since we don't have a tracing profiler, this
gives a rough approximation of that. You have to
manually insert trace points, but it is still
helpful.

I used this in investigating some test performance,
and felt it would be useful to keep around for
future performance investigation.

R=dimitern, TheMue
CC=
https://codereview.appspot.com/7537044

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