os.Stat: returned error on nonexistent path did not satisfy IsNotExist

Add direct test for the problem to make the fix commit clearer.

Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.

src/os/os_anyos_test.go

@@ -4,6 +4,7 @@ package os_test
 
 import (
 	. "os"
+	"path/filepath"
 	"strconv"
 	"testing"
 	"time"
@@ -31,6 +32,16 @@ func TestMkdir(t *testing.T) {
 	}
 }
 
+func TestStatBadDir(t *testing.T) {
+	dir := TempDir()
+	badDir := filepath.Join(dir, "not-exist/really-not-exist")
+	_, err := Stat(badDir)
+	// TODO: PathError moved to io/fs in go 1.16; fix next line once we drop go 1.15 support.
+	if pe, ok := err.(*PathError); !ok || !IsNotExist(err) || pe.Path != badDir {
+		t.Errorf("Mkdir error = %#v; want PathError for path %q satisifying IsNotExist", err, badDir)
+	}
+}
+
 func writeFile(t *testing.T, fname string, flag int, text string) string {
 	f, err := OpenFile(fname, flag, 0666)
 	if err != nil {