Fixed PHP-978: Standalone connection to arbiter fails
Improve tests by adding one for replicaset connection to arbiter node

I've removed the logs for the first connection search, as this is basically
random. The code for it (in php_mongo_connect) has as comment:

   /* We don't care about the result so although we assign it to a var, we
    * only do that to handle errors and return it so that the calling function
    * knows whether a connection could be obtained or not. */

The new test also illustrates that it works just fine connecting to the arbiter
node in replicaset connection mode.