encoding/asn1: fix test on OpenBSD

encoding/asn1: fix test on OpenBSD

time.Parse uses time.Local if it has the right zone offset,
otherwise it calls time.FixedZone.  The test's use of reflect.DeepEqual
meant that the test expected time.FixedZone always, failing
when the local time zone really would have used -0700 for
that time.  The fix is to format the time to display only the
pieces we intend to test.

R=golang-dev, agl, iant
CC=golang-dev
https://golang.org/cl/5437088