Tests: Make the beforeunload event tests work regardless of extensions

Some browser extensions, like React DevTools, send messages to the content area.
Since our beforeunload event test listens for all messages, it used to catch
those as well, failing the test.

Add a `source` field to the payload JSON and check for it before treating the
message as coming from our own test to make sure the test passes even with such
browser extensions installed.

Closes gh-5478

test/data/event/onbeforeunload.html

@@ -4,6 +4,7 @@
 	<script>
 		function report( event ) {
 			var payload = {
+				source: "jQuery onbeforeunload iframe test",
 				event: event.type
 			};
 			return parent.postMessage( JSON.stringify( payload ), "*" );

test/unit/event.js

@@ -1448,13 +1448,22 @@ QUnit[ /(ipad|iphone|ipod)/i.test( navigator.userAgent ) ? "skip" : "test" ](
 	var done = assert.async();
 
 	window.onmessage = function( event ) {
+		try {
 			var payload = JSON.parse( event.data );
 
+			// Ignore unrelated messages
+			if ( payload.source === "jQuery onbeforeunload iframe test" ) {
 				assert.ok( payload.event, "beforeunload", "beforeunload event" );
 
 				iframe.remove();
 				window.onmessage = null;
 				done();
+			}
+		} catch ( e ) {
+
+			// Messages may come from other sources, like browser extensions;
+			// some may not be valid JSONs and thus cannot be `JSON.parse`d.
+		}
 	};
 
 	iframe.appendTo( "#qunit-fixture" );