[Tests] Add array return type for provider methods

Author: OskarStark

src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToRfc3339TransformerTest.php

@@ -49,12 +49,12 @@ public static function allProvider(): array
         ];
     }
 
-    public static function transformProvider()
+    public static function transformProvider(): array
     {
         return self::allProvider();
     }
 
-    public static function reverseTransformProvider()
+    public static function reverseTransformProvider(): array
     {
         return array_merge(self::allProvider(), [
             // format without seconds, as appears in some browsers
@@ -132,7 +132,7 @@ public function testReverseTransformExpectsValidDateString($date)
         $transformer->reverseTransform($date);
     }
 
-    public static function invalidDateStringProvider()
+    public static function invalidDateStringProvider(): array
     {
         return [
             'invalid month' => ['2010-2010-01'],