Audit and convert tests to test/unit (minitest)

:construction: WIP :construction: 

This PR converts the RSpec tests to minitest. Most of the tests were translated over, but I'll make inline notes where I modified or removed tests for clarity.
@mtodd is there a reason we instrument both `bind.net_ldap` in Net::LDAP, and then instrument `bind.net_ldap_connection` in Net::LDAP::Connection again? Running into some weirdness with this stubbing, and and I'd like to consolidate this to a test for Net::LDAP::Connection if possible.