fix: #3597692 Move DRUPAL_TEST_IN_CHILD_SITE out of DrupalKernel::bootEnvironment() and share it with DrupalApplication

Derek Wrightrequested to merge
issue/drupal-3597692:3597692-dr-set-drupal_test_in_child_site into main

fix: #3597692 Move DRUPAL_TEST_IN_CHILD_SITE out of DrupalKernel::bootEnvironment() and share it with DrupalApplication

Closes #3597692

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