ISAICP-8958: Disable transitions during behat.

web/modules/custom/joinup_test/joinup_test.libraries.yml

@@ -3,3 +3,11 @@ testing.js_errors_log:
   header: true
   js:
     /core/modules/system/tests/modules/js_testing_log_test/js/js_testing_log.js: { weight: -1000 }
+
+# Relies on core css_disable_transitions_test.
+testing.css_disable_transitions:
+  css:
+    theme:
+      /core/modules/system/tests/modules/css_disable_transitions_test/css/disable_transitions.theme.css: { weight: 100 }
+  js:
+    /core/modules/system/tests/modules/css_disable_transitions_test/js/disable_transitions.theme.js: { }

web/modules/custom/joinup_test/joinup_test.module

@@ -14,6 +14,7 @@ function joinup_test_page_attachments(array &$attachments): void {
   // @todo Inject this conditionally, when behat is active.
   // Attach JS errors collector (used in behat).
   $attachments['#attached']['library'][] = 'joinup_test/testing.js_errors_log';
+  $attachments['#attached']['library'][] = 'joinup_test/testing.css_disable_transitions';
 }
 
 /**