The test just doubled a certain number of times
and then gave up. On a mostly fast but occasionally
slow machine this may never make the test run
long enough to see the linear growth.

Change test to keep doubling until the first round
takes at least a full second, to reduce the effect of
occasional scheduling or other jitter.

The failure we saw had a time for the first round
of around 100ms.

Note that this test still passes once it sees a linear
effect, even with a very small total time.
The timeout here only applies to how long the execution
must be to support a reported failure.

LGTM=khr
R=khr
CC=golang-codereviews, rlh
https://golang.org/cl/164070043