[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 

Yup, you understand it perfectly. It shouldn't happen, it would be an exceptional case. Didn't feel right to blow up, though.