Fix PHPStan level 6 errors in tests

tests/Behat/Bootstrap/FeatureContext.php

@@ -76,6 +76,9 @@ public static function clean(AfterSuiteScope $scope)
 
     private mixed $lastReturn;
 
+    /**
+     * @var array<mixed>
+     */
     private array $lastReturnAsArray;
 
     public function __construct(string $redmineVersion)

tests/Fixtures/AssertingHttpClient.php

@@ -35,8 +35,12 @@ public static function create(TestCase $testCase, array $dataSet, ...$dataSets):
         return $client;
     }
 
-    private $testCase;
-    private $client;
+    private TestCase $testCase;
+    private HttpClient $client;
+
+    /**
+     * @var array<mixed>
+     */
     private $fifoStack = [];
 
     private function __construct(TestCase $testCase, HttpClient $client)

tests/Fixtures/MockClient.php

@@ -16,7 +16,7 @@ final class MockClient implements Client
 {
     use ClientApiTrait;
 
-    public static function create()
+    public static function create(): self
     {
         return new self();
     }
@@ -35,9 +35,9 @@ public static function create()
      */
     public $useOriginalGetMethod = false;
 
-    public $responseBodyMock;
-    public $responseCodeMock;
-    public $responseContentTypeMock;
+    public string $responseBodyMock;
+    public int $responseCodeMock;
+    public string $responseContentTypeMock;
 
     private function __construct() {}