Issue #2542318 by neclimdul: Clean up local variables in MigrateTestBase in tearDown

core/modules/migrate/src/Tests/MigrateTestBase.php

@@ -80,6 +80,7 @@ protected function setUp() {
   protected function tearDown() {
     Database::removeConnection('migrate');
     parent::tearDown();
+    unset($this->migration, $this->migrateMessages, $this->collectMessages, $this->databaseDumpFiles);
   }
 
   /**