skip sha512 tests when it's not available

This makes the entire testsuite run through on HHVM 3.8.0 on phpunit
beta-2015-06-18 :-)

Note: this does not mean that DokuWiki is fully compatible with HHVM,
but it's a good step!

_test/tests/inc/auth_password.test.php

@@ -16,9 +16,16 @@ class auth_password_test extends DokuWikiTest {
         'kmd5'  => 'a579299436d7969791189acadd86fcb716',
         'djangomd5'  => 'md5$abcde$d0fdddeda8cd92725d2b54148ac09158',
         'djangosha1' => 'sha1$abcde$c8e65a7f0acc9158843048a53dcc5a6bc4d17678',
-        'sha512' => '$6$abcdefgh12345678$J9.zOcgx0lotwZdcz0uulA3IVQMinZvFZVjA5vapRLVAAqtay23XD4xeeUxQ3B4JvDWYFBIxVWW1tOYlHX13k1'
+
     );
 
+    function __construct() {
+        if(defined('CRYPT_SHA512') && CRYPT_SHA512 == 1) {
+            // Check SHA512 only if available in this PHP
+            $this->passes['sha512'] = '$6$abcdefgh12345678$J9.zOcgx0lotwZdcz0uulA3IVQMinZvFZVjA5vapRLVAAqtay23XD4xeeUxQ3B4JvDWYFBIxVWW1tOYlHX13k1';
+        }
+    }
+
 
     function test_cryptPassword(){
         foreach($this->passes as $method => $hash){