Add tests for wc_sanitize_order_id()

This commit adds four test cases (via a PHPUnit data provider) for the `wc_sanitize_order_id()` function.

While writing the tests, it was also discovered that the function was returning a *string*, not an integer as indicated by the DocBlock, so explicit type-casting was added.

includes/wc-order-functions.php

@@ -886,7 +886,7 @@ add_action( 'woocommerce_cancel_unpaid_orders', 'wc_cancel_unpaid_orders' );
  * @param int $order_id Order ID.
  */
 function wc_sanitize_order_id( $order_id ) {
-	return filter_var( $order_id, FILTER_SANITIZE_NUMBER_INT );
+	return (int) filter_var( $order_id, FILTER_SANITIZE_NUMBER_INT );
 }
 add_filter( 'woocommerce_shortcode_order_tracking_order_id', 'wc_sanitize_order_id' );
 

tests/unit-tests/order/class-wc-tests-order-functions.php

@@ -1271,6 +1271,18 @@ class WC_Tests_Order_Functions extends WC_Unit_Test_Case {
 		$this->assertEquals( $expected, $orders );
 	}
 
+	/**
+	 * Test wc_sanitize_order_id().
+	 *
+	 * @testWith ["123", 123]
+	 *           ["#123", 123]
+	 *           ["# 123", 123]
+	 *           ["Order #123", 123]
+	 */
+	public function test_wc_sanitize_order_id( $id, $expected ) {
+		$this->assertSame( $expected, wc_sanitize_order_id( $id ) );
+	}
+
 	/**
 	 * Test wc_get_order_note().
 	 *