time: always run ZoneAbbr test

CL 52430 added logic to skip the testZoneAbbr test in locales where
the timezone does not have a three-letter name, because the following
line

  Parse(RFC1123, t1.Format(RFC1123))

failed for timezones with only numeric names (like -07).

Since Go 1.11, Parse supports the parsing of timezones with numeric
names (this was implemented in CL 98157), so we can now run the test
unconditionally.

Change-Id: I8ed40e1ba325c0c0dc79c4184a9e71209e2e9a02
Reviewed-on: https://go-review.googlesource.com/127757


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>