[fenix] Fix BrowserRobot.goToHomescreen to wait for home screen to be rendered

It was previously just waiting for the application to be idle, which
is not a reliable check in case the navigation to home is triggered
on completion of an async operation i.e., the app / main thread may
be idle, but the navigation to home hasn't completed yet.

goToHomeScreen further needs to wait for the browser fragment's
home button to exist before clicking it.

app/src/androidTest/java/org/mozilla/fenix/ui/robots/BrowserRobot.kt

@@ -605,10 +605,19 @@ class BrowserRobot {
         }
 
         fun goToHomescreen(interact: HomeScreenRobot.() -> Unit): HomeScreenRobot.Transition {
+            assertTrue(
+                mDevice.findObject(UiSelector().description("Home screen"))
+                    .waitForExists(waitingTime)
+            )
+
             onView(withContentDescription("Home screen"))
                 .check(matches(isDisplayed()))
                 .click()
-            mDevice.waitForIdle()
+
+            assertTrue(
+                mDevice.findObject(UiSelector().resourceId("$packageName:id/homeLayout"))
+                    .waitForExists(waitingTime)
+            )
 
             HomeScreenRobot().interact()
             return HomeScreenRobot.Transition()