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.
Make sure we can talk to an arbiter with the rest of the nodes down.

It's near impossible to make a reliable automated test for this. I've tested
the following manually:

- make servers
- put all the secondary nodes in recovery mode with replSetMaintenance: 1
- step down the primary
- put the remaining (now secondary node) in recovery mode

Then requesting a standalone connection to the arbiter, or a replicaset
connection to the arbiter work just fine. Of course, upon writing I get a "not
master" error (which apparently changed code in 2.6 from 10058 to 100).
Added another test case.

This test case shows that even with the connection being made against an
arbiter with MONGO_RP_ANY, the read preferences are correctly respected for
normal queries (write, and read).
Merged pull request #689