[TEST] Fix/Remove Tests

The properties are now implemented by PHPs DOM, keep the existing
tests just for verification.

tests/FluentDOM/DOM/Node/NonDocumentTypeChildNodeTest.php

@@ -115,30 +115,16 @@ public function testSetNextElementChildExpectingException(): void {
       $document = new Document();
       $document->loadXML('<foo><!--comment--></foo>');
       $this->expectException(\Error::class);
-      $this->expectExceptionMessage(
-        'Cannot write read-only property FluentDOM\\DOM\\Comment::$nextElementSibling'
-      );
       $document->documentElement->firstChild->nextElementSibling = $document->createElement('foo');
     }
 
     public function testSetPreviousElementChildExpectingException(): void {
       $document = new Document();
       $document->loadXML('<foo><!--comment--></foo>');
       $this->expectException(\Error::class);
-      $this->expectExceptionMessage(
-        'Cannot write read-only property FluentDOM\\DOM\\Comment::$previousElementSibling'
-      );
       $document->documentElement->firstChild->previousElementSibling = $document->createElement('foo');
     }
 
-    public function testSetUnknownProperty(): void {
-      $document = new Document();
-      $document->loadXML('<foo><!--comment--></foo>');
-      $node = $document->documentElement->firstChild;
-      $this->expectException(\ErrorException::class);
-      $node->SOME_PROPERTY = 'success';
-    }
-
     public function testIssetUnknownPropertyExpectingFalse(): void {
       $document = new Document();
       $document->loadXML('<foo><!--comment--></foo>');