WebGPU CTS: Don't return the promise from async_test

This is not needed and should only be done for promise_test.

Change-Id: I53aaee9d8f500c85a7d74be0f92ffcb6164b2424
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436654
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811897}

third_party/blink/web_tests/wpt_internal/webgpu/common/runtime/wpt.js

@@ -48,7 +48,6 @@ function addWPTTests(testcases) {
       });
 
       running.push(p);
-      return p;
     };
 
     async_test(wpt_fn, name);