Switch loadedNotes test to listen for the event rather than using s…

…etTimeout

Much better to just listen for the thing we are trying to test,
than to wait 50ms and see whether the thing had been called.
I think this was causing the test to fail occasinally on CI.  (It passes locally.)

test/browser/services/OsmService.test.js

@@ -722,14 +722,12 @@ describe('OsmService', () => {
         headers: { 'Content-Type': 'text/xml' }
       });
 
-      _osm.on('loadedNotes', spy);
-      _osm.loadNotes({ /*no options*/ });
-
-      window.setTimeout(() => {
-        // was: calledOnce, now called multiple times as we fetch margin tiles
-        expect(spy.called).to.be.ok;
+      _osm.on('loadedNotes', () => {
+        expect(fetchMock.calls()).to.have.lengthOf.at.least(1);
         done();
-      }, 50);
+      });
+
+      _osm.loadNotes({ /*no options*/ });
     });
   });