cmd/go: parallelize some tests

Cuts tests from 35 to 25 seconds.

Many of these could be parallel if the test runner were modified to
give each test its own workdir cloned from the tempdir files they
use. But later. This helps for now.

Updates #17751

Change-Id: Icc2ff87cca60a33ec5fd8abb1eb0a9ca3e85bf95
Reviewed-on: https://go-review.googlesource.com/32850
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>