fix: error handling when login via test env (#7214)

no issue

- When you choose a non existent user (email+pwd) for login via the test env, no error was shown.

core/test/utils/index.js

@@ -550,6 +550,10 @@ login = function login(request) {
                 client_id: 'ghost-admin',
                 client_secret: 'not_available'
             }).then(function then(res) {
+                if (res.statusCode !== 200) {
+                    return reject(res.body);
+                }
+
                 resolve(res.body.access_token);
             }, reject);
     });