Added DatabaseConnectionPool.close()

lib/__mocks__/DatabaseConnectionPool.js

@@ -21,9 +21,14 @@ const mockRunQuery = jest.fn((sql, params) => {
 	};
 });
 
+const mockClose = jest.fn();
+
 jest.mock('./DatabaseConnectionPool', () => {
 	return jest.fn().mockImplementation(() => {
-		return { runQuery: mockRunQuery };
+		return {
+			runQuery: mockRunQuery,
+			close: mockClose
+		};
 	});
 });