Don't set test fail status in web-extensions-helper.js. (#61149)

web-extension-helper.js was overriding the test status to just
`test.FAIL`, leaving out the test message. This causes a test failure
to look like: `FAIL message: Undefined.UNDEFINED` since the message and
stack are `undefined`.

`test.set_status(status, message, stack);` is already being handled by
testharness.js in the `step()` method (see line 2878) so we can just
rely on that.

Author: justinlulejian

resources/web-extensions-helper.js

@@ -27,10 +27,6 @@ globalThis.runTestsWithWebExtension = function(extensionPath) {
 
     test.done();
 
-    if (!data.result) {
-      test.set_status(test.FAIL);
-    }
-
     if (data.remainingTests) {
       return;
     }