Fix missing methods in null tester

devtools/src/tester/null.rs

@@ -1,6 +1,7 @@
 use crate::Program;
 use crate::core::window;
 use crate::core::{Element, Theme};
+use crate::futures::Subscription;
 use crate::runtime::Task;
 use crate::widget::horizontal_space;
 
@@ -23,6 +24,10 @@ impl<P: Program> Tester<P> {
         Self { _type: PhantomData }
     }
 
+    pub fn is_idle(&self) -> bool {
+        true
+    }
+
     pub fn is_busy(&self) -> bool {
         false
     }
@@ -35,6 +40,10 @@ impl<P: Program> Tester<P> {
         Task::none()
     }
 
+    pub fn subscription(&self, _program: &P) -> Subscription<Tick<P>> {
+        Subscription::none()
+    }
+
     pub fn view<'a, T: 'static>(
         &'a self,
         _program: &P,