Use Sorbet compatible setup-initializations in tests

sorbet_compatible_test_init.md

Use Sorbet compatible setup-initializations in tests

Sorbet can help with type inference in tests if the setup is right. Otherwise types will be reduced to T.untyped.

Don't do

class Test < ActiveSupport::TestCase
  def setup # Don't use `dev setup`.
    @user = make_fixture(User, :alex) # Don't forget `T.let`
  end
end

Do

class Test < ActiveSupport::TestCase
  setup do
    @user = T.let(make_fixture(User, :alex), User)
  end
end

With this latter @user will be infered as type User, and it does not need to be nilable.