cmd/vet: add a test for embedded stringer

This should help narrowing down the possible cause of #20514.

Updates #20514.

Change-Id: Ie997400c9749aace7783bd585b23dbb4cefc181d
Reviewed-on: https://go-review.googlesource.com/44375Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/testdata/print.go

@@ -87,6 +87,9 @@ func PrintfTests() {
 	fmt.Printf("%s", &stringerv)
 	fmt.Printf("%v", &stringerv)
 	fmt.Printf("%T", &stringerv)
+	fmt.Printf("%s", &embeddedStringerv)
+	fmt.Printf("%v", &embeddedStringerv)
+	fmt.Printf("%T", &embeddedStringerv)
 	fmt.Printf("%v", notstringerv)
 	fmt.Printf("%T", notstringerv)
 	fmt.Printf("%q", stringerarrayv)
@@ -123,6 +126,8 @@ func PrintfTests() {
 	fmt.Printf("%X", 2.3)                      // ERROR "arg 2.3 for printf verb %X of wrong type"
 	fmt.Printf("%s", stringerv)                // ERROR "arg stringerv for printf verb %s of wrong type"
 	fmt.Printf("%t", stringerv)                // ERROR "arg stringerv for printf verb %t of wrong type"
+	fmt.Printf("%s", embeddedStringerv)        // ERROR "arg embeddedStringerv for printf verb %s of wrong type"
+	fmt.Printf("%t", embeddedStringerv)        // ERROR "arg embeddedStringerv for printf verb %t of wrong type"
 	fmt.Printf("%q", notstringerv)             // ERROR "arg notstringerv for printf verb %q of wrong type"
 	fmt.Printf("%t", notstringerv)             // ERROR "arg notstringerv for printf verb %t of wrong type"
 	fmt.Printf("%t", stringerarrayv)           // ERROR "arg stringerarrayv for printf verb %t of wrong type"
@@ -346,6 +351,14 @@ func (*stringer) Warnf(int, string, ...interface{}) string {
 	return "warnf"
 }
 
+type embeddedStringer struct {
+	foo string
+	stringer
+	bar int
+}
+
+var embeddedStringerv embeddedStringer
+
 type notstringer struct {
 	f float64
 }