tests: ensure all database usages are properly closed

Leaving lots of DB connections open seems to make OpenJDK crash a lot,
giving false fails for the CI tests.

app/src/test/java/org/fdroid/fdroid/data/DatabaseMigration.java

@@ -38,6 +38,7 @@ public class DatabaseMigration {
         Preferences.setupForTests(context);
         SQLiteOpenHelper opener = new MigrationRunningOpenHelper(context);
         opener.getReadableDatabase();
+        opener.close();
     }
 
     /**