fix(testing): re-add Puppeteer asElement() calls

Fixes a regression caused by the removal of a few `asElement()` calls. Turns out these are necessary for the case where the handle is `null`. To avoid type errors, we cast the returned value to be of type `ElementHandle<Element>` rather than `ElementHandle<Node`.

STENCIL-1032: `E2EPage.find()` throws error when using piercing selector targeting a non-existing element

Fixes: #5113

src/testing/puppeteer/puppeteer-element.ts

@@ -593,7 +593,7 @@ async function findWithCssSelector(
       return null;
     }
 
-    elmHandle = shadowHandle;
+    elmHandle = shadowHandle.asElement() as puppeteer.ElementHandle<Element>;
   }
 
   return elmHandle;
@@ -646,7 +646,7 @@ async function findWithText(
   );
 
   if (jsHandle) {
-    return jsHandle;
+    return jsHandle.asElement() as puppeteer.ElementHandle<Element>;
   }
 
   return null;