API phpunit 9 support

composer.json

@@ -15,12 +15,12 @@
     ],
     "require": {
         "silverstripe/vendor-plugin": "^1.0",
-        "silverstripe/framework": "^4.0",
+        "silverstripe/framework": "^4.10",
         "silverstripe/cms": "^4.0",
         "silverstripe/versioned": "^1.0"
     },
     "require-dev": {
-        "sminnee/phpunit": "^5.7",
+        "phpunit/phpunit": "^9",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "extra": {

phpunit.xml.dist

@@ -1,8 +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/WidgetControllerTest.php

@@ -15,7 +15,7 @@ class WidgetControllerTest extends FunctionalTest
         TestWidget::class,
     ];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -32,7 +32,7 @@ class WidgetControllerTest extends FunctionalTest
         $response = $this->get($page->URLSegment);
 
         $formAction = sprintf('%s/widget/%d/%s', $page->URLSegment, $widget->ID, 'Form');
-        $this->assertContains(
+        $this->assertStringContainsString(
             $formAction,
             $response->getBody(),
             "Widget forms are rendered through WidgetArea templates"
@@ -47,12 +47,12 @@ class WidgetControllerTest extends FunctionalTest
         $this->get($page->URLSegment);
         $response = $this->submitForm('Form_Form', null, array('TestValue' => 'Updated'));
 
-        $this->assertContains(
+        $this->assertStringContainsString(
             'TestValue: Updated',
             $response->getBody(),
             "Form values are submitted to correct widget form"
         );
-        $this->assertContains(
+        $this->assertStringContainsString(
             sprintf('Widget ID: %d', $widget->ID),
             $response->getBody(),
             "Widget form acts on correct widget, as identified in the URL"