all tests currently share the same platform string and fail to
vet expected platforms

Fixes #19958

Change-Id: I2801e1e84958e31975769581e27ea5ca6a0edf5b
Reviewed-on: https://go-review.googlesource.com/40511
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>