refactor: replace deprecated 'String.prototype.substr()' (#4852)

'substr()' is deprecated, so we replace it with 'slice()' which works similarily.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

lib/reporters/html.js

@@ -181,7 +181,7 @@ function HTML(runner, options) {
       if (indexOfMessage === -1) {
         stackString = test.err.stack;
       } else {
-        stackString = test.err.stack.substr(
+        stackString = test.err.stack.slice(
           test.err.message.length + indexOfMessage
         );
       }