API phpunit 9 support

composer.json

@@ -11,10 +11,11 @@
         }
     ],
     "require": {
+        "php": "^7.3 || ^8.0",
         "silverstripe/cms": "^4.0"
     },
     "require-dev": {
-        "sminnee/phpunit": "^5.7",
+        "phpunit/phpunit": "^9.5",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "suggest": {

phpunit.xml.dist

@@ -1,7 +1,9 @@
 <phpunit bootstrap="vendor/silverstripe/cms/tests/bootstrap.php" colors="true">
-    <testsuite name="Default">
-        <directory>tests/</directory>
-    </testsuite>
+    <testsuites>
+        <testsuite name="Default">
+            <directory>tests/</directory>
+        </testsuite>
+    </testsuites>
     <filter>
         <whitelist addUncoveredFilesFromWhitelist="true">
             <directory suffix=".php">src/</directory>

tests/ExternalLinksTest.php

@@ -22,7 +22,7 @@ class ExternalLinksTest extends SapphireTest
         ExternalLinksTestPage::class
     );
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();