time: make the ParseInLocation test more robust

time: make the ParseInLocation test more robust

The tzdata 2017a update (2017-02-28) changed the abbreviation of the
Asia/Baghdad time zone (used in TestParseInLocation) from 'AST' to the
numeric '+03'.

Update the test so that it skips the checks if we're using a recent
tzdata release.

Fixes #19457

Change-Id: I45d705a5520743a611bdd194dc8f8d618679980c
Reviewed-on: https://go-review.googlesource.com/37964
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Gbp-Pq: Name cl-37964--tzdata-2017a.patch