check.php: random subdomain for /ht/add-subdomain

jobs/check.php

@@ -289,7 +289,7 @@ exit
 	}
 
 	{
-		$ht_subdomain = 'test3';
+		$ht_subdomain = bin2hex(random_bytes(16));
 
 		startTest('subdomain site creation');
 		curlTest('/ht/add-subdomain', [