diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 8123d08..936a88d 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -271,7 +271,6 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
 
   $command = array(
     $phpunit_bin,
-    '-c ' . \Drupal::root() . '/core',
     '--log-junit',
     escapeshellarg($phpunit_file),
   );
@@ -294,10 +293,16 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
     ));
   }
 
+  // Need to change directories before running the command so that we can use
+  // relative paths in the configuration file's exclusions.
+  $old_cwd = getcwd();
+  chdir(\Drupal::root() . "/core");
+
   // exec in a subshell so that the environment is isolated when running tests
   // via the simpletest UI.
-  $ret = exec(join($command, ' '), $output, $status);
+  $ret = exec(join($command, " "), $output, $status);
 
+  chdir($old_cwd);
   putenv('SIMPLETEST_DB=');
   return $ret;
 }