Call flush_effects in test context update()

In gpui1 we used to do this even outside of top-level contexts, but not
sure we should make tests work that differently to the main app.

crates/gpui2/src/app/test_context.rs

@@ -64,8 +64,8 @@ impl TestAppContext {
     }
 
     pub fn update<R>(&self, f: impl FnOnce(&mut AppContext) -> R) -> R {
-        let mut lock = self.app.lock();
-        f(&mut *lock)
+        let mut cx = self.app.lock();
+        cx.update(f)
     }
 
     pub fn read_window<R>(

crates/gpui2/src/platform/test/dispatcher.rs

@@ -77,7 +77,6 @@ impl TestDispatcher {
             type Output = ();
 
             fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-                eprintln!("self.count: {}", self.count);
                 if self.count > 0 {
                     self.count -= 1;
                     cx.waker().wake_by_ref();