Fix DatePeriod tests for implementing IteratorAggregate

I wrote them in a wrong way in vimeo#8312, so they are rewritten now as @AndrolGenhald advised in vimeo#8327

tests/CoreStubsTest.php

@@ -2,11 +2,13 @@
 
 namespace Psalm\Tests;
 
+use Psalm\Tests\Traits\InvalidCodeAnalysisTestTrait;
 use Psalm\Tests\Traits\ValidCodeAnalysisTestTrait;
 
 class CoreStubsTest extends TestCase
 {
     use ValidCodeAnalysisTestTrait;
+    use InvalidCodeAnalysisTestTrait;
 
     /**
      * @return iterable<string,array{string,assertions?:array<string,string>,error_levels?:string[]}>
@@ -86,23 +88,29 @@ public function providerValidCodeParse(): iterable
             'error_levels' => [],
             'php_version' => '8.0',
         ];
-        yield 'DatePeriod implements only Traversable on PHP 7' => [
+        yield 'DatePeriod implements IteratorAggregate on PHP 8' => [
             '<?php
-
             $period = new DatePeriod("R4/2012-07-01T00:00:00Z/P7D");
-            if ($period instanceof IteratorAggregate) {}',
+            $period->getIterator();',
             'assertions' => [],
             'error_levels' => [],
-            'php_version' => '7.3',
+            'php_version' => '8.0',
         ];
-        yield 'DatePeriod implements IteratorAggregate on PHP 8' => [
-            '<?php
+    }
 
+    /**
+     * @return iterable<string,array{string,error_message:string,1?:string[],2?:bool,3?:string}>
+     */
+    public function providerInvalidCodeParse(): iterable
+    {
+        yield 'DatePeriod implements only Traversable on PHP 7' => [
+            '<?php
             $period = new DatePeriod("R4/2012-07-01T00:00:00Z/P7D");
-            if ($period instanceof IteratorAggregate) {}',
-            'assertions' => [],
-            'error_levels' => ['RedundantCondition'],
-            'php_version' => '8.0',
+            $period->getIterator();',
+            'error_message' => 'blahblah',
+            'error_levels' => [],
+            'strict_mode' => false,
+            'php_version' => '7.3',
         ];
     }
 }