diff -u b/core/tests/Drupal/Setup/Commands/TestTeardownCommand.php b/core/tests/Drupal/Setup/Commands/TestTeardownCommand.php
--- b/core/tests/Drupal/Setup/Commands/TestTeardownCommand.php
+++ b/core/tests/Drupal/Setup/Commands/TestTeardownCommand.php
@@ -76,6 +76,8 @@
    *   The database URL.
    * @param $app_root
    *   The app root.
+   *
+   * @see \Drupal\Tests\BrowserTestBase::cleanupEnvironment()
    */
   public function teardown($db_prefix, $db_url, $app_root) {
     // Connect to the test database.
diff -u b/core/tests/Drupal/Tests/Setup/Commands/SetupDrupalTestScriptTest.php b/core/tests/Drupal/Tests/Setup/Commands/SetupDrupalTestScriptTest.php
--- b/core/tests/Drupal/Tests/Setup/Commands/SetupDrupalTestScriptTest.php
+++ b/core/tests/Drupal/Tests/Setup/Commands/SetupDrupalTestScriptTest.php
@@ -43,12 +43,17 @@
     $phpBinaryFinder = new PhpExecutableFinder();
     $phpBinaryPath = $phpBinaryFinder->find();
 
+    // Create a connection to the DB configured in SIMPLETEST_DB.
+    $connection = Database::getConnection('default', $this->addTestDatabase(''));
+    $table_count = count($connection->schema()->findTables('%'));
+
     $command_line = $phpBinaryPath . ' core/scripts/setup-drupal-test.php setup-drupal-test --setup_class "this-class-does-not-exist" --db_url "' . getenv('SIMPLETEST_DB') . '"';
     $process = new Process($command_line, $this->root);
     $process->run();
 
     $this->assertContains('There was a problem loading this-class-does-not-exist', $process->getErrorOutput());
     $this->assertSame(1, $process->getExitCode());
+    $this->assertSame($table_count, count($connection->schema()->findTables('%')), 'No additional tables created in the database');
   }
 
   /**
@@ -58,12 +63,17 @@
     $phpBinaryFinder = new PhpExecutableFinder();
     $phpBinaryPath = $phpBinaryFinder->find();
 
+    // Create a connection to the DB configured in SIMPLETEST_DB.
+    $connection = Database::getConnection('default', $this->addTestDatabase(''));
+    $table_count = count($connection->schema()->findTables('%'));
+
     $command_line = $phpBinaryPath . ' core/scripts/setup-drupal-test.php setup-drupal-test --setup_class "' . static::class . '" --db_url "' . getenv('SIMPLETEST_DB') . '"';
     $process = new Process($command_line, $this->root);
     $process->run();
 
     $this->assertContains('You need to define a class implementing \Drupal\Setup\TestSetupInterface', $process->getErrorOutput());
     $this->assertSame(1, $process->getExitCode());
+    $this->assertSame($table_count, count($connection->schema()->findTables('%')), 'No additional tables created in the database');
   }
 
   /**
@@ -95,7 +105,7 @@
     $key = $this->addTestDatabase($db_prefix);
     $this->assertGreaterThan(0, count(Database::getConnection('default', $key)->schema()->findTables('%')));
     $test_database = new TestDatabase($db_prefix);
-    $test_file = $test_database->getTestSitePath() . DIRECTORY_SEPARATOR . '.htkey';
+    $test_file = $this->root . DIRECTORY_SEPARATOR . $test_database->getTestSitePath() . DIRECTORY_SEPARATOR . '.htkey';
     $this->assertFileExists($test_file);
 
     // Install another site so we can ensure tear down only removes one site at
@@ -120,7 +130,7 @@
     // Ensure the other sites tables and files still exist.
     $this->assertGreaterThan(0, count(Database::getConnection('default', $other_key)->schema()->findTables('%')));
     $test_database = new TestDatabase($other_db_prefix);
-    $test_file = $test_database->getTestSitePath() . DIRECTORY_SEPARATOR . '.htkey';
+    $test_file = $this->root . DIRECTORY_SEPARATOR . $test_database->getTestSitePath() . DIRECTORY_SEPARATOR . '.htkey';
     $this->assertFileExists($test_file);
 
     // Now test the tear down process as well.