Lower-casing of PHPT file names can break tests

[cmb69]

Consider following CI.phpt:

--TEST--
Testing case-(in)sensitivity of filenames
--FILE--
<?php
echo basename(__FILE__);
?>
--EXPECT--
CI.php

This test currently fails with the following diff:

+ci.php
-CI.php

Obviously, we must not lower-case the PHP file names, to avoid breaking tests which expect the filenames (or parts of these). This has likely gone unnoticed so far due to #79, but will hit us as soon as that issue is resolved.