Test form #states

Here is something to start with.
If you look at \Drupal\form_test\Form\FormTestStatesForm::$field_types you'll see a lot of disabled field types with some comments explaining why.

BEWARE!
The test passed because some field types are not tested. In fact, #states does not work as expected.
You can manually see what happens by enabling the form_test module and go to form-test/states then check the "trigger" checkbox and look at fields above.

Very small progress.

Just for the record, here is my last patch updated to remove all the composer and phpunit related stuff.