Task #3405392: Opt-in to all extended testing within the shared gitlab-ci templates

Closes #3405392

.gitlab-ci.yml

@@ -20,5 +20,16 @@ include:
 #
 # Docs at https://git.drupalcode.org/project/gitlab_templates/-/blob/1.0.x/includes/include.drupalci.variables.yml
 ################
-# variables:
-#   SKIP_ESLINT: '1'
+variables:
+    OPT_IN_TEST_PREVIOUS_MINOR:
+        value: '1'
+
+    OPT_IN_TEST_NEXT_MINOR:
+        value: '1'
+
+    OPT_IN_TEST_PREVIOUS_MAJOR:
+        value: '1'
+
+    OPT_IN_TEST_MAX_PHP:
+        value: '1'
+