Skip .NET tests if mscoree is not available (#14281)

ext/com_dotnet/tests/bug73679.phpt

@@ -5,6 +5,7 @@ com_dotnet
 --SKIPIF--
 <?php
 if (PHP_INT_SIZE != 8) die("skip this test is for 64bit platforms only");
+if (!class_exists("dotnet")) die("skip mscoree not available");
 ?>
 --FILE--
 <?php

ext/com_dotnet/tests/bug77177.phpt

@@ -2,6 +2,10 @@
 Bug #77177 (Serializing or unserializing COM objects crashes)
 --EXTENSIONS--
 com_dotnet
+--SKIPIF--
+<?php
+if (!class_exists("dotnet")) die("skip mscoree not available");
+?>
 --FILE--
 <?php
 $com = new COM("WScript.Shell");