Further tests improvements

* Extract common mimic tests to isolated/shared
* Get rid from minitest-around
* Introduce rails_after and rails_before mimic tests
* Give tests proper names
  * MimicAlone – when json is never required
  * MimicDefine – simple test to check if JSON is properly defined
Mimic: fix everything around me

* Define JSON::State as empty class to be compatible with ActiveSupport
* Move Object#to_json definition to a non-verbose block
* Fix test_object failing test

Dolla-dolla bill y'all
Mimic: correct files mode
Merge pull request #156 from krasnoukhov/better-tests

Further tests improvements