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.

- convert rspec tests to test/unit
- rename 'test/common' to 'test/test_helper'
- remove metaid dependency
- remove rspec dependency
- remove autotest
- auto generate gemspec files manifest

:+1: I knew of that problem, but the way I got around that was by having the caller add the load path:

```sh
time ruby -Itest test/test_ldap_connection.rb
```

I think I formed this habit from Ruby 1.8 days. Since this library doesn't support 1.8, I'm fine with switching to `require_relative`. [Ways to load code](https://practicingruby.com/articles/ways-to-load-code) was a good read on the subject too.