Replace `assert` with `assert_equal`
Replace `assert` with `assert_equal`

`Minitest::Assertions#assert(test, msg = nil)` check if
first argument (`test`) is not falsy value, and second argument (`msg`)
is used as a custom error message.
`assert 'bar', tes.col` always passes because first arguemtn (`'bar'`) is truthy.

amatsuda/stateful_enum,amatsuda/stateful_enum