testing: use strings.LastIndexAny in (*common).decorate

Change-Id: I14c521f4e599e14c71a52342b0ead5d0d4007f1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/527655
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Kirill Kolyshkin <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: tklauser

src/testing/testing.go

@@ -771,9 +771,7 @@ func (c *common) decorate(s string, skip int) string {
 	if file != "" {
 		if *fullPath {
 			// If relative path, truncate file name at last file name separator.
-		} else if index := strings.LastIndex(file, "/"); index >= 0 {
-			file = file[index+1:]
-		} else if index = strings.LastIndex(file, "\\"); index >= 0 {
+		} else if index := strings.LastIndexAny(file, `/\`); index >= 0 {
 			file = file[index+1:]
 		}
 	} else {