[CI] Run specs and tests

While we move away from rspec and towards minitest, now seems like a good time to start running both to make sure builds don't break.

cc @jch @schaary 

I was running into problems with this in https://github.com/ruby-ldap/ruby-net-ldap/pull/121, so I fixed it in that branch on 1018963e80d118f628fda1b35aae1eebd1f963f7