Update .gitlab-ci.yml

Closes #3447974

.gitlab-ci.yml

@@ -49,9 +49,9 @@ include:
 # Docs at https://git.drupalcode.org/project/gitlab_templates/-/blob/1.0.x/includes/include.drupalci.variables.yml
 ################
 variables:
-  _PHPUNIT_CONCURRENT: '1'
-  _PHPUNIT_EXTRA: ''
-  SKIP_PHPUNIT: '0'
+  _PHPUNIT_CONCURRENT: '0'
+  _PHPUNIT_EXTRA: '-c $CI_PROJECT_DIR/$_WEB_ROOT/core --testdox --colors=always'
+  OPT_IN_TEST_PREVIOUS_MINOR: '0'
   OPT_IN_TEST_NEXT_MINOR: '1'
   OPT_IN_TEST_NEXT_MAJOR: '1'
 
@@ -60,7 +60,6 @@ phpunit:
   variables:
     SYMFONY_DEPRECATIONS_HELPER: "ignoreFile=$CI_PROJECT_DIR/$_WEB_ROOT/core/.deprecation-ignore.txt"
 
-
 ###################################################################################
 #
 #                                        *