Update idlharness test for webtransport

Catch promise rejections to avoid the test timing out.

Bug: 1201569
Change-Id: Ia70e362aa43b5c36ee3d33ffe40e1ffcfcdb98f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144590
Reviewed-by: Nidhi Jaju <[email protected]>
Commit-Queue: Kenichi Ishibashi <[email protected]>
Cr-Commit-Position: refs/heads/main@{#918718}

Author: bashi

webtransport/idlharness.any.js

@@ -16,5 +16,9 @@ idl_test(
       // ReceiveStream
     });
     self.webTransport = new WebTransport("https://example.com/");
+    // `ready` and `closed` promises will be rejected due to connection error.
+    // Catches them to avoid unhandled rejections.
+    self.webTransport.ready.catch(() => {});
+    self.webTransport.closed.catch(() => {});
   }
 );