Error: Call to undefined function Drupal\deferjs\Form\drupal_flush_all_caches() in test coverage

Problem/Motivation

Getting error while running test coverage 1) Drupal\Tests\deferjs\Unit\Form\DeferJsFormTest::testSubmitForm
Error: Call to undefined function Drupal\deferjs\Form\drupal_flush_all_caches()

Steps to reproduce

Download the default branch code, having test coverage and run the test coverage.
Code coverage is 100% but getting below error
1) Drupal\Tests\deferjs\Unit\Form\DeferJsFormTest::testSubmitForm
Error: Call to undefined function Drupal\deferjs\Form\drupal_flush_all_caches()

Proposed resolution

Handling drupal_flush_all_caches() method in test coverage and raising a PR for the above issue.