[CI] Add integration tests for Net::LDAP#open behavior

I'm working through some ill-defined behavior (discussed [here](https://github.com/github/github-ldap/commit/620ce324bb82ca9109dfb5adc6bfa1b058328443)) with `Net::LDAP#open` and wanted to reproduce the problematic behavior in the most minimal way possible.

This PR just starts by adding some basic integration tests for `Net::LDAP#open` behavior (and not even good integration tests, at that).

I'll be following up with a reproduction of the bad behavior (hopefully).

cc @jch 

To double check I understand this. This clears PDU's we've `queue_read` from the socket for this particular search keyed by `message_id` that weren't consumed. I think this is good cleanup, but when do we hit this scenario? Is this an exceptional case?