Enable PHPUnit job for auto_updates and package_manager unit tests only

Closes #3409774

.gitlab-ci.yml

@@ -54,8 +54,6 @@ variables:
   SKIP_PHPCS: '1'
   SKIP_PHPSTAN: '1'
   SKIP_STYLELINT: '1'
-  _PHPUNIT_EXTRA: '--testsuite=unit'
-  _PHPUNIT_TESTGROUPS: 'auto_updates,package_manager'
   _WEB_ROOT: '.'
 
 ###################################################################################
@@ -126,10 +124,10 @@ composer:
     - test -d ./core/modules/package_manager
     - test -d ./core/modules/auto_updates
     - rm -r -f ./modules/contrib
-    - mkdir ./modules/custom
-    - ln -s ./modules/custom/$CI_PROJECT_NAME $PWD/core/modules
 
 phpunit:
   extends: .phpunit-base
   variables:
+    _PHPUNIT_CONCURRENT: '1'
+    _PHPUNIT_EXTRA: '--types PHPUnit-Unit --module package_manager'
     _WEB_ROOT: ''