Don't skip tests which are supposed to fail; mark them as xfail (#15472)

Especially regarding buggy server behavior, we should not skip those
tests, because it is unlikely that fixes to the server's behavior will
even be noticed.  Instead we mark these tests as xfail, so we get a
warning if the test succeeds, and can act appropriately.

ext/mysqli/tests/mysqli_change_user_get_lock.phpt

@@ -5,8 +5,9 @@ mysqli
 --SKIPIF--
 <?php
 require_once 'skipifconnectfailure.inc';
-die("skip - is the server still buggy?");
 ?>
+--XFAIL--
+The server is still buggy
 --INI--
 max_execution_time=240
 --FILE--

ext/mysqli/tests/mysqli_change_user_locks_temporary.phpt

@@ -5,8 +5,9 @@ mysqli
 --SKIPIF--
 <?php
 require_once 'skipifconnectfailure.inc';
-die("skip - is the server still buggy?");
 ?>
+--XFAIL--
+The server is still buggy
 --FILE--
 <?php
     require_once 'connect.inc';

ext/mysqli/tests/mysqli_stmt_attr_get_prefetch.phpt

@@ -5,8 +5,9 @@ mysqli
 --SKIPIF--
 <?php
 require_once 'skipifconnectfailure.inc';
-die("SKIP: prefetch isn't supported at the moment");
 ?>
+--XFAIL--
+prefetch isn't supported at the moment
 --FILE--
 <?php
     require 'table.inc';

ext/mysqli/tests/mysqli_stmt_result_metadata_sqltests.phpt

@@ -5,9 +5,9 @@ mysqli
 --SKIPIF--
 <?php
 require_once 'skipifconnectfailure.inc';
-
-die("skip Check again when the Klingons visit earth - http://bugs.mysql.com/bug.php?id=42490");
 ?>
+--XFAIL--
+http://bugs.mysql.com/bug.php?id=42490
 --FILE--
 <?php
     require 'table.inc';