The testZoneAbbr assumes that

  Parse(RFC1123, t1.Format(RFC1123))

will always succeed. This is not true because Format will fall back to
the numeric zone (ex. -07) for timezones with no abbreviation, but
Parse won't accept the numeric zone when the layout specifies 'MST'
(an abbreviation).

Skip the zone abbreviation test in timezones with no abbreviation.

Fixes #21183

Change-Id: If04691cc23ae1075d8a953733024e17f5a7646de
Reviewed-on: https://go-review.googlesource.com/52430
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>