decoder: tests: When decode result is unexpected - report it with full details

that is with %#v instead of %q. Because e.g. %q for nil []byte will be
printing still as "", not nil etc. When some decode test fails here we
want to get full information, not some pretty one.

ogorek_test.go

@@ -79,7 +79,7 @@ func TestDecode(t *testing.T) {
 		}
 
 		if !reflect.DeepEqual(v, test.expected) {
-			t.Errorf("%s: got\n%q\n expected\n%q", test.name, v, test.expected)
+			t.Errorf("%s: decode:\nhave: %#v\nwant: %#v", test.name, v, test.expected)
 		}
 
 		v, err = dec.Decode()