Migrate from Minitest to RSpec

[jodosha]

A few rules to follow:

• If you want to work on this, please leave a comment here. We want to avoid many people to work on the same task.

• We expect this migration to happen within a week from now.

• Do not change the tests, only migrate from Minitest to RSpec syntax. If not sure, please ask here.

• Do not remove the old tests, only add the new ones.

• Make CI build to use RSpec tests.

• Target develop branch, instead of master.

• All the tests artefacts like fixtures, goes under spec/support.

• Try to reflect the structure under lib/. Eg. lib/hanami/view.rb should be tested under spec/hanami/view_spec.rb, while test/rendering_test.rb should go under spec/integration/rendering_spec.rb.

If you want to take inspiration, please have a look at: hanami/mailer#67

[katafrakt]

I will take it, if free.

[jodosha]

@katafrakt Please go for it. Thanks.

[jodosha]

Implemented by #74