Expose rake tasks for Rails `rake -T` + Organize rake tasks

Currently, the minitest rake tasks are not showing when I do `rake -T` in a Rails project, because the descriptions of the tasks are commented.

Also `rake test`, available in `rake -T`, doesn't actually run all tests upon inspection. 
They only run tests in controllers, helpers, mailers, integration and models. So I have tests in test/lib which were not ran when I do a `rake test`, and that's unexpected.

This commit hopes to fix all these:

- expose rake tasks to `rake -T`in Rails by uncommenting `desc`
- `rake minitest` should run all tasks instead of just default tasks
- remove ` default_tasks` as it is obsolete because of `all_tasks`
- remove `rake test` and `rake minitest:all` since they are essentially the same as `rake minitest`