xerrors: avoid infinite loop in test (detailed).Error method

Fixes golang/go#40536

Change-Id: I5daaec48301cdedb2efcfadaf1dcd41cb8e96b01
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/246640
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>

Author: Bryan C. Mills

fmt_test.go

@@ -395,7 +395,7 @@ var _ xerrors.Formatter = detailed{}
 
 type detailed struct{}
 
-func (e detailed) Error() string { return fmt.Sprint(e) }
+func (e detailed) Error() string { panic("should have called FormatError") }
 
 func (detailed) FormatError(p xerrors.Printer) (next error) {
 	p.Printf("out of %s", "peanuts")