net/mail: don't use MDT in test

When time.Parse sees a timezone name that matches the local timezone,
it uses the local timezone. The tests weren't expecting that,
so using MDT broke with TZ=America/Boise (where MDT means Mountain
Daylight Time). Just use GMT instead.

Fixes #43354

Change-Id: Ida70c8c867e2568b1535d1dfbf1fb0ed9e0e5c1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/280072


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>