🌿 Fix error message in test

@std/async/deadline changes it's internal implementation and the error
has changed from `DeadlineError` to `DOMException` with `Signal timed
out` error originated from `AbortSignal.timeout()`.

tests/denops/testutil/shared_server_test.ts

@@ -110,7 +110,7 @@ Deno.test("useSharedServer()", async (t) => {
         await useSharedServer({ timeout: 0 });
       },
       Error,
-      "Deadline",
+      "Signal timed out",
     );
   });
 });