The return values of the LoadLocation are inherently dependent
on the runtime environment. Add LoadLocationFromTZData, whose
results depend only on the timezone data provided as arguments.

Fixes #20629

Change-Id: I43b181f4c05c219be3ec57327540263b7cb3b2aa
Reviewed-on: https://go-review.googlesource.com/68890Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>