add webauthn authentication js tests

notably i had to change `window.location = foo` to
`window.location.assign` so that i could have something to spy on with
jest. mocking sucks. Otherwise this is pretty similar to the
registerSecurityKey.test.js file.

app/assets/javascripts/authenticateSecurityKey.js

@@ -47,7 +47,7 @@
                   throw Error(response.statusText);
                 })
                 .then(data => {
-                  window.location = data.redirect_url;
+                  window.location.assign(data.redirect_url);
                 });
             })
             .catch(error => {