Add custom mock assertions

[clarkedb]

Could add minitest assertions used with objects

• assert_mocked
• assert_mock_called
• assert_mock_called_with (maybe)

[clarkedb]

It looks like this may be more difficult than originally expected. A plugin cannot define the assertions. They have to be injected into a reopened Minitest::Assertions which would require the project using grift to modify it's test case base class.