remove rougue execute permission on tests
RUBY-529 raise exception for multiple attempts to auth on the same db

Server side, MongoDB will log out the first user when a second authentication
is issued for the same database. In the past we've handled this by just quietly
updating the authentication credentials we have stored for that db.

The driver spec for auth now requires that we raise an explicit exception so
that users don't accidentally log out previous authenications applied to the
database unknowingly.

Previous this would have been allowed, but now this raises an exception:

```ruby
client.add_auth('test', 'joe', 'password')
client.add_auth('test', 'jane', 'password')
```