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

`assert_predicate` isn't supported in `test/unit` for 1.9, so this breaks the 1.9 build too.

I think this is another good reason to use the minitest gem and `Minitest::Test` instead of `Test::Unit::TestCase` and the bundled minitest, though perhaps I'm overstating the benefits.