Additional test for disabled function re-declare

[Ayesh]

Related to: #5473
Bug: 79382

For disabled functions, there is currently a test:

--INI--
disable_functions=strlen
--FILE--
<?php

function strlen(string $x): int {
    $len = 0;
    while (isset($x[$len])) $len++;
    return $len;
}

var_dump(strlen("foobar"));

?>
--EXPECT--
int(6)

This test passes, but I think PHP is still using the internal strlen implementation here because even if I change the user-land function declaration to return another value, the test still passes.

This PR adds a new test that re-declares disbaled substr function (which has no internal optimizations).

Thank you 🙏🏿.

[nikic]

Nice catch! I've fixed this issue in 2af1d36.

With that done, I don't think we need the extra test for substr(), as the strlen() case now also works as it is supposed to.