nuageinit: Fix tests

Commit 07d17ca189fcf3cc44b7706040b05ca8135c3b85 set the recommended
permissions for the SSH authorized keys file and directory. The tests,
however, were failing on CI.

Use stat to check for the proper permissions.

Fixes: 07d17ca189f nuageinit: Set recommended SSH permissions
Reported by: Jenkins

(cherry picked from commit 8edd6c07c8dafcc5828bceb5fea0684c7d0d0775)