Add unit test

tests/PHPUnit/WcSubscriptions/Helper/SubscriptionHelperTest.php

@@ -0,0 +1,37 @@
+<?php
+declare(strict_types=1);
+
+namespace WooCommerce\PayPalCommerce\WcSubscriptions\Helper;
+
+use Mockery;
+use WC_Order;
+use WC_Subscription;
+use WooCommerce\PayPalCommerce\TestCase;
+use function Brain\Monkey\Functions\when;
+
+class SubscriptionHelperTest extends TestCase
+{
+	public function testPreviousTransaction()
+	{
+		$subscription = Mockery::mock(WC_Subscription::class);
+		$subscription->shouldReceive('get_related_orders')
+			->andReturn(
+				[
+					1 => 1,
+					3 => 3,
+					2 => 2,
+				]
+			);
+
+		$wc_order = Mockery::mock(WC_Order::class);
+		$wc_order->shouldReceive('get_status')->andReturn('processing');
+		$wc_order->shouldReceive('get_transaction_id')->andReturn('ABC123');
+
+		when('wc_get_order')->justReturn($wc_order);
+
+		$this->assertSame(
+			'ABC123',
+			(new SubscriptionHelper())->previous_transaction($subscription)
+		);
+	}
+}