add gitlabci

Fixed failed phpunit test cases.

Thanks
Samit K.

This change does a lot more than just add gitlabci. :)

This commit breaks sites running 9.x and 10.x prior to 10.3, since it uses the new Base classes introduced in 10.3.

Since the deprecated classes won't actually be removed until D12, seems those changes should reverted.

Specifically:

-use Drupal\Core\Render\Element\FormElement;
+use Drupal\Core\Render\Element\FormElementBase;

and

-class Interval extends FormElement {
+class Interval extends FormElementBase {

Then, a new version that only supports 10.3 or later can be created with these changes.

@larowlan

I can create an MR, yes.