Issue #3415403: Opt GitLab CI into always testing against the previous minor of core

.gitlab-ci.yml

@@ -51,6 +51,8 @@ include:
 variables:
   _PHPUNIT_CONCURRENT: '1'
   _PHPUNIT_TESTGROUPS: ''
+  # Always test against the previous minor version of core.
+  OPT_IN_TEST_PREVIOUS_MINOR: '1'
 #   SKIP_ESLINT: '1'
 
 
@@ -118,6 +120,12 @@ composer:
     - git clone https://git.drupalcode.org/project/drupal.git --depth 1 --branch $CORE_BRANCH /tmp/drupal-core
     - mv /tmp/drupal-core/composer $_WEB_ROOT
 
+composer (previous minor):
+  before_script:
+    - !reference [composer, before_script]
+  after_script:
+    - !reference [composer, after_script]
+
 phpcs:
   before_script:
     # Use core's PHPCS configuration.
@@ -152,3 +160,7 @@ phpunit:
     - when: on_success
   variables:
     _PHPUNIT_EXTRA: '--types PHPUnit-$TEST_TYPE --module $MODULE'
+
+phpunit (previous minor):
+  rules:
+    - !reference [phpunit, rules]