We recently added these large zip64 tests. They're slow-ish already,
but fast enough in non-race mode with t.Parallel. But in race mode,
the concurrency makes them much slower than the normal
non-race-to-race multiplier.

They're taking so long now that it's causing test failures when it
sometimes is over the test timeout threshold.

Change-Id: I02f4ceaa9d6cab826708eb3860f47a57b05bdfee
Reviewed-on: https://go-review.googlesource.com/33423
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>