cmd/nm: fix testGoLib helper to be module-agnostic

Updates #30228

Change-Id: I3c7864e6725312df5ec978cdc130ccfe8fc2e738
Reviewed-on: https://go-review.googlesource.com/c/162836
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/nm/nm_test.go

@@ -222,12 +222,16 @@ func testGoLib(t *testing.T, iscgo bool) {
 	if e := file.Close(); err == nil {
 		err = e
 	}
+	if err == nil {
+		err = ioutil.WriteFile(filepath.Join(libpath, "go.mod"), []byte("module mylib\n"), 0666)
+	}
 	if err != nil {
 		t.Fatal(err)
 	}
 
 	args := []string{"install", "mylib"}
 	cmd := exec.Command(testenv.GoToolPath(t), args...)
+	cmd.Dir = libpath
 	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
 	out, err := cmd.CombinedOutput()
 	if err != nil {