support cross version phpunit (#92)

tests/CoreTest.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0.
+ */
+use AWS\CRT\CRT;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
+
+require_once('common.inc');
+
+// This intentionally does not inherit from CrtTestCase because it needs a clean-room environment
+final class CoreTest extends TestCase {
+
+    // The CRT should always be available in this test suite
+    public function testIsAvailable() {
+        $this->assertTrue(CRT::isAvailable());
+    }
+
+    // We have done nothing to necessitate loading the CRT, it should not be loaded
+    public function testIsLoaded() {
+        $this->assertTrue(!CRT::isLoaded());
+    }
+}