Test: don't unmount `MatrixChat` before setup is finished (#25783)

test/app-tests/loading-test.tsx

@@ -592,6 +592,11 @@ describe("loading:", function () {
 
                     return tokenLoginCompletePromise;
                 })
+                .then(() => {
+                    return expectAndAwaitSync().catch((e) => {
+                        throw new Error("Never got /sync after login: did the client start?");
+                    });
+                })
                 .then(() => {
                     // check that the localstorage has been set up in such a way that
                     // the reloaded app can pick up where we leave off.