Change PDO to singleton instance

- Add SQLite throwaway DB implementation
- Add integration tests for Custom class

tests/integration/SubDocument.php

@@ -0,0 +1,11 @@
+<?php declare(strict_types=1);
+
+namespace Test\Integration;
+
+/**
+ * A sub-document for testing
+ */
+class SubDocument
+{
+    public function __construct(public string $foo = '', public string $bar = '') { }
+}