wait for next sync before inspecting memberlist

before we needed a 10s delay here to make the test work
reliable, this should be faster in the best case.

src/scenarios/lazy-loading.js

@@ -102,7 +102,8 @@ async function joinCharliesWhileAliceIsOffline(alice, charly6to10) {
         await member6.talk("where is charly?");
     }
     member6.log.unmute().done();
-    await delay(1000);
+    const catchupPromise = alice.waitForNextSuccessfulSync();
     await alice.setOffline(false);
-    await delay(1000);
+    await catchupPromise;
+    await delay(2000);
 }

src/session.js

@@ -161,6 +161,33 @@ module.exports = class RiotSession {
         });
     }
 
+    waitForSyncResponseWith(predicate) {
+        return this.page.waitForResponse(async (response) => {
+            if (response.request().url().indexOf("/sync") === -1) {
+                return false;
+            }
+            return predicate(response);
+        });
+    }
+
+    /** wait for a /sync request started after this call that gets a 200 response */
+    async waitForNextSuccessfulSync() {
+        const syncUrls = [];
+        function onRequest(request) {
+            if (request.url().indexOf("/sync") !== -1) {
+                syncUrls.push(request.url());
+            }
+        }
+
+        this.page.on('request', onRequest);
+
+        await this.page.waitForResponse((response) => {
+            return syncUrls.includes(response.request().url()) && response.status() === 200;
+        });
+
+        this.page.removeListener('request', onRequest);
+    }
+
     goto(url) {
         return this.page.goto(url);
     }

src/usecases/timeline.js

@@ -64,10 +64,7 @@ module.exports.receiveMessage = async function(session, expectedMessage) {
     if (isExpectedMessage) {
         assertMessage(lastMessage, expectedMessage);
     } else {
-        await session.page.waitForResponse(async (response) => {
-            if (response.request().url().indexOf("/sync") === -1) {
-                return false;
-            }
+        await session.waitForSyncResponseWith(async (response) => {
             const body = await response.text();
             if (expectedMessage.encrypted) {
                 return body.indexOf(expectedMessage.sender) !== -1 &&