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>

Author: thanm

src/cmd/go/internal/work/cover.go

@@ -44,6 +44,9 @@ func BuildActionCoverMetaFile(runAct *Action) (string, error) {
 		}
 		if pred.Package.ImportPath == p.ImportPath {
 			metaFile := pred.Objdir + covcmd.MetaFileForPackage(p.ImportPath)
+			if cfg.BuildN {
+				return metaFile, nil
+			}
 			f, err := os.Open(metaFile)
 			if err != nil {
 				return "", err

src/cmd/go/testdata/script/list_n_cover.txt

@@ -0,0 +1,20 @@
+# Adding -cover to "go test -n" should not cause failures,
+# see issue 67952. In the regular (no "-n") case for an
+# empty package test action for the package will look for
+# a static meta-data file produced by the cover tool
+# during the build action; when "-n" is in effect that
+# meta-data file doesn't exist, so the code that reads
+# the meta-data file has to be stubbed out.
+
+go test -vet=off -n -cover ./f
+
+-- go.mod --
+module M
+
+go 1.21
+-- f/f.go --
+package f
+
+func Id() int {
+     return 42
+}