test: add test helper for accessibility

As proposed in #22343, the components can be automatically validated
for accessibility. This adds a test helper that runs Axe to check
for accessibility violations.

typings/jest.d.ts

@@ -1,5 +1,6 @@
 declare namespace jest {
   interface Matchers<R> {
     toMatchRenderedSnapshot(): R;
+    toHaveNoViolations(): R;
   }
 }