Running the testsuite with mysqli:// and with Xdebug loaded (1) generates an "E" at every test with a message like:
mysqli_close(): Couldn't fetch mysqli

Without a master/slave configuration, the connection is initiated once and closed twice by the eZMySQL(i?)DB::close() function which causes a PHP error.

The error doesn't appear using mysql:// for the reason that an issue exist with the mysql functions and copying the link by reference (which is still the case in /lib/ezdb/classes/ezmysqldb.php:96):

The following code snippet will not issue an error

$c = mysql_connect('localhost', 'root', '*****' );
$d =& $c;
$e = $d;

mysql_close( $d );
mysql_close( $c );

Commenting: $e = $d and running this code again will give you the (expected) error:
Warning: mysql_close(): 4 is not a valid MySQL-Link resource in /home/patrick.allaert/test.php on line 7

(1) the fact that it doesn't produce an error without Xdebug inside ezp code is completely unknown. The above test case works consistently with or without Xdebug loaded.