After a failure on the build dashboard I tested testcarchive test 2 and
found that it failed an average of 1 in 475 runs on my laptop.  With
this change it ran over 50,000 times without failing.  I bumped up the
other iteration limits to correspond.

Change-Id: I0155c68161a2c2a09ae25c91e9269f1e8702628d
Reviewed-on: https://go-review.googlesource.com/18309
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>