Forms with randomised element ids don't support JS validation

Test the fix for client-side validation in forms with randomised ids:

1. Place 'testform.php' in your web root and access it
2. Open browser dev tools -> console and make sure 'persist logs' is enabled (or equivalent). Leave the console open.
3. Submit the form empty
4. Verify you don't see any JS errors in the console
5. Verify you see a form validation message 'This element is required'.
6. Submit the form again, watching for a browser post. Verify nothing is posted.
7. Reload the page to clear the validation error
8. Click in the text field
9. Tab away
10. Verify you see a form validation message 'This element is required'.

 Regression test 1: Confirming this works for forms without randomised ids (the vast majority of cases in Moodle):

1. Edit 'testform.php' and change:

   ['data-random-ids' => 1]

   to:

   ['data-random-ids' => 0]

   thus disabling the randomised ids.

2. Access the form (or reload it if it's already open)
3. Make sure 'persist logs' is enabled in your dev tools console (or its equivalent). Leave the console open.
4. Submit the form empty
5. Verify you don't see any JS errors in the console
6. Verify you see a form validation message 'This element is required'.
7. Submit the form again, watching for a browser post. Verify nothing is posted.
8. Reload the page to clear the validation error
9. Click in the text field
10. Tab away
11. Verify you see a form validation message 'This element is required'.

Regression test 2: Confirming client side validation works when multiple forms are used and when randomisation of ids is enabled

N.B. This test is based on the test in MDL-65217, but adds client side validation test steps.

1. Place 'testmultipleforms.php' in your web root and access it
2. Open browser dev tools -> console and make sure 'persist logs' is enabled (or equivalent). Leave the console open.
3. Submit the first form empty
4. Verify you don't see any JS errors in the console
5. Verify you see a form validation message 'This element is required'.
6. Submit the form again, watching for a browser post. Verify nothing is posted.
7. Reload the page to clear the validation error
8. Select a course from the first form's autocomplete (if you don't have any courses, create a few in your site and retry)
9. Now, submit the second form
10. Verify you see a form validation message 'This element is required' for the second form.
11. Now, Verify you can now select THE SAME course from the second form's autocomplete.