cmd/go: fix "go test -n -cover" glitch with no-test packages
Invoking "go test -n -cover ./..." on a collection of packages that
includes at least one package with code but no tests can result in
spurious error of the form
my/package: open $WORK/b112/covmeta.
b07a5f2dff1231cae3a6bdd70c8cc7c19da16abf8ac59747d8e9859c03594d37: no such file or directory
This patch fixes this issue by ensuring that we stub out some of the
meta-data file handling for no-test packages if "-n" is in effect.
Fixes #67952.
Change-Id: Ic6160c275abdec5e5b8beecc6a59accb2b8cfe7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/592201
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>