test: ensure message event fires in worker message port test

Add assertion to verify that the MessagePort's message event is
actually emitted in test-worker-message-port-infinite-message-loop.js.
Previously, the test could pass even if the event was not fired.

PR-URL: #59885
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Jarred-Sumner

test/parallel/test-worker-message-port-infinite-message-loop.js

@@ -27,3 +27,14 @@ port2.postMessage(0);
 // This is part of the test -- the event loop should be available and not stall
 // out due to the recursive .postMessage() calls.
 setTimeout(common.mustCall(), 0);
+
+// Assert that the 'message' handler was actually called.
+//
+// We do not want to assert a specific call count, so common.mustCall cannot be
+// used in the port1.on('message' callback directly.
+process.once(
+  'beforeExit',
+  common.mustCall(() => {
+    assert(count > 0, 'count should be greater than 0');
+  })
+);