Add a phpunit extension to flag slow tests

See https://aaronsaray.com/2021/finding-slow-tests-in-phpunit-9/

plugins/woocommerce/phpunit.xml

@@ -49,4 +49,7 @@
 			<file>./includes/wc-widget-functions.php</file>
 		</exclude>
 	</coverage>
+	<extensions>
+		<extension class="Automattic\WooCommerce\Tests\SlowTests" />
+	</extensions>
 </phpunit>

plugins/woocommerce/tests/legacy/bootstrap.php

@@ -287,6 +287,9 @@ class WC_Unit_Tests_Bootstrap {
 		require_once $this->tests_dir . '/framework/traits/trait-wc-rest-api-complex-meta.php';
 		require_once dirname( $this->tests_dir ) . '/php/helpers/HPOSToggleTrait.php';
 		require_once dirname( $this->tests_dir ) . '/php/helpers/SerializingCacheTrait.php';
+
+		// Extensions.
+		require_once dirname( $this->tests_dir ) . '/php/helpers/SlowTests.php';
 	}
 
 	/**

plugins/woocommerce/tests/php/helpers/SlowTests.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Automattic\WooCommerce\Tests;
+
+use Automattic\Jetpack\IdentityCrisis\Exception;
+use PHPUnit\Runner\AfterTestHook;
+
+/**
+ *
+ */
+class SlowTests implements AfterTestHook {
+	/**
+	 * Threshold of what's considered "slow".
+	 */
+	protected const MAX_SECONDS_ALLOWED = 3;
+
+	/**
+	 *
+	 *
+	 * @param string $test Name of the test.
+	 * @param float  $time Elapsed time in seconds of the test.
+	 *
+	 * @return void
+	 */
+	public function executeAfterTest( string $test, float $time ): void {
+		if ( $time > self::MAX_SECONDS_ALLOWED ) {
+			fwrite( STDERR, sprintf( "\nThe %s test took %s seconds!\n", $test, $time ) );
+		}
+	}
+}