Rails upgrade: rake test does nothing

After upgrading my rails app to 4.2, I started writing automated tests.

I started by doing something super simple:

class SimpleTest < ActionDispatch::IntegrationTest
  test 'Browse a page' do
    assert true
    get '/'
    assert_response :success
  end
end

But when I:

▶ rake test
▶ bin/rake test

Nothing happens. No error, no blocking, process finishes, just nothing.

Did I miss something?