Make button selector more explicit

frontend/test/e2e/guest.test.js

@@ -13,7 +13,8 @@ module.exports = {
       .url(browser.globals.url)
       .waitForElementVisible('body')
       .click("link text", "Login")
-      .assert.containsText('.btn', 'Login with OAS')
+      .waitForElementVisible('.login-button')
+      .assert.containsText('.login-button', 'Login with OAS')
       .end();
   }
 };