Make testharness.js escape all backslashes in strings, not just the 1st

Change-Id: I0059ca822f3d492be3d8175c8ea0e2fe51fa2cfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932367
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#732361}

resources/testharness.js

@@ -1019,7 +1019,7 @@ policies and contribution forms [3].
 
         switch (typeof val) {
         case "string":
-            val = val.replace("\\", "\\\\");
+            val = val.replace(/\\/g, "\\\\");
             for (var p in replacements) {
                 var replace = "\\" + replacements[p];
                 val = val.replace(RegExp(String.fromCharCode(p), "g"), replace);