cmd/go: update TestScripts/svn to not depend on TZ database

cmd/go: update TestScripts/svn to not depend on TZ database

`TestScripts/svn` test suite fails if the host does not have a TZ
database installed.

This CL updates those tests so SVN formats dates using UTC, which
don't require a TZ database.

Fixes #56527

Change-Id: I20f3c03c3cedd7d748f4623dddc66bd04d1df318
Reviewed-on: https://go-review.googlesource.com/c/go/+/447335
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>