#3370952: Allow for concurrency via a variable.

Fixes #3370952

includes/include.drupalci.main.yml

@@ -307,8 +307,13 @@ nightwatch:
     # Provide some context on the test run.
     - vendor/bin/drush status
     # Finally, execute tests.
-    - sudo -u www-data -E vendor/bin/phpunit --no-interaction --printer="\Drupal\Tests\Listeners\HtmlOutputPrinter" --bootstrap $PWD/web/core/tests/bootstrap.php $_WEB_ROOT/modules/custom --log-junit junit.xml $_PHPUNIT_EXTRA
+    - |
+      [[ $_PHPUNIT_CONCURRENT == "1" ]] && sudo -u www-data -E vendor/bin/paratest --functional -v --bootstrap $PWD/web/core/tests/bootstrap.php $_WEB_ROOT/modules/custom --log-junit junit.xml $_PHPUNIT_EXTRA
+    - |
+      [[ $_PHPUNIT_CONCURRENT == "0" ]] && sudo -u www-data -E vendor/bin/phpunit --no-interaction --printer="\Drupal\Tests\Listeners\HtmlOutputPrinter" --bootstrap $PWD/web/core/tests/bootstrap.php $_WEB_ROOT/modules/custom --log-junit junit.xml $_PHPUNIT_EXTRA
+  after_script:
     - cp /var/log/apache2/test.apache.access.log $CI_PROJECT_DIR/apache.access.log.txt
+    - cp /var/log/apache2/test.apache.error.log $CI_PROJECT_DIR/apache.error.log.txt
   artifacts:
     expire_in: 6 mos
     expose_as: 'junit-browser_output-apache_log'
@@ -318,6 +323,7 @@ nightwatch:
     paths:
       - junit.xml
       - apache.access.log.txt
+      - apache.error.log.txt
       - $BROWSERTEST_OUTPUT_DIRECTORY
 
 phpunit: