Missing use statement in Drupal\form_test\Form\FormTestVerticalTabsForm

Problem/Motivation

Discovered via PHPStan in #3178534: Start running PHPStan on Drupal core (level 0 - part 1):

  Line   core/modules/system/tests/modules/form_test/src/Form/FormTestVerticalTabsForm.php  
 ------ ----------------------------------------------------------------------------------- 
  57     Call to static method encode() on an unknown class Drupal\form_test\Form\Json.     

This is a test form that is never actually submitted so the code is never exercised, but we need a submitForm() method and it might as well be syntactically correct in case we use it in the future.

Steps to reproduce

Proposed resolution

Add the missing use statement.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet