test: added test case showing empty waitFor

Author: jrparish

tests/lib/rules/prefer-find-by.test.ts

@@ -51,7 +51,10 @@ ruleTester.run(RULE_NAME, rule, {
       code: `
         await waitFor(() => expect(${queryMethod}('baz')).toBeInTheDocument());
       `
-    }))
+    })),
+    {
+      code: `await waitFor()`
+    }
   ],
   invalid: [
     // using reduce + concat 'cause flatMap is not available in node10.x