Provide assertions ivar for Minitest.

Minitest requires an assertion var be available to increment inside
the class that it is running. Add a module that can be mixed in that
provides the methods required.

See https://github.com/seattlerb/minitest/commit/714cd8a7e0e9d319c6bf45cfedd0910f381b267b
Merge pull request #1 from rspec/lint-for-rails-master

Provide assertions getter/setter for Minitest.