time: skip a often-flaky test in short mode
time: skip a often-flaky test in short mode

In -test.short mode, skip measuring the upper bound of time
sleeps. The API only guarantees minimum bounds on sleeps,
anyway, so this isn't a bug we're ignoring as much as it is
simply observing bad builder virtualization and/or loaded
machines.

We keep the test in full mode where developers will
presumably be running on a lightly-loaded, native, fast
machine.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5713044

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