ocsp: fix test flakiness

The test was assuming that the OCSP creation wouldn't cross a minute
boundary, which is flakey nonsense. Instead assert that the timestamp in
the OCSP response is within an hour of when we check it.

Change-Id: I3c822b738e9b04385319eb29a1cd275dc2ed112e
Reviewed-on: https://go-review.googlesource.com/36648
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>