cmd/go: parallellize more test cases

Sprinkle in some more calls to tg.Parallel in the cmd/go
tests. These are the easy cases that don't operate in the
testdata directory.

I think the best thing to do for those tests that do operate
in testdata is to use the script tests. They're easier to
read and write, and jump into t.Parallel immediately.

Updates #17751

Change-Id: If8aeb8129cfdc0a9aa91f5a540f179790077fce5
Reviewed-on: https://go-review.googlesource.com/c/go/+/212618


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>