fmt: refined tests for non-string error verbs

This is a refinement of CL 164557.

Make it explicit in tests that using a non-string verb with
fmtError does not result in falling back to using fmt.Formatter.

Change-Id: I6d090f31818eb7cc7668d7565b1449c91cd03a23
Reviewed-on: https://go-review.googlesource.com/c/go/+/164701
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/fmt/errors_test.go

@@ -353,6 +353,10 @@ func TestErrorFormatter(t *testing.T) {
 		err:  intError(4),
 		fmt:  "%d",
 		want: "4",
+	}, {
+		err:  intError(4),
+		fmt:  "%🤪",
+		want: "%!🤪(fmt_test.intError=4)",
 	}}
 	for i, tc := range testCases {
 		t.Run(fmt.Sprintf("%d/%s", i, tc.fmt), func(t *testing.T) {
@@ -446,6 +450,12 @@ type intError int
 
 func (e intError) Error() string { return fmt.Sprint(e) }
 
+func (e wrapped) Format(w fmt.State, r rune) {
+	// Test that the normal fallback handling after handleMethod for
+	// non-string verbs is used. This path should not be reached.
+	fmt.Fprintf(w, "Unreachable: %d", e)
+}
+
 func (e intError) FormatError(p errors.Printer) (next error) {
 	p.Printf("error %d", e)
 	return nil