Add signal setter test for refs.

Author: joshwilsonvu

docs/reactivity.md

@@ -698,6 +698,11 @@ function Component() {
   );
 }
 
+function Component() {
+  const [canvas, setCanvas] = createSignal();
+  return <canvas ref={(c) => setCanvas(c)} />;
+}
+
 function createCustomStore() {
   const [store, updateStore] = createStore({});
 

test/rules/reactivity.test.ts

@@ -240,6 +240,10 @@ export const cases = run("reactivity", rule, {
         }} />
       );
     }`,
+    `function Component() {
+      const [canvas, setCanvas] = createSignal();
+      return <canvas ref={c => setCanvas(c)} />;
+    }`,
     // mapArray()
     `function createCustomStore() {
       const [store, updateStore] = createStore({});