Make MSVCRT memory leak checking usable for the test suite

Make MSVCRT memory leak checking usable for the test suite

While basic support for MSVCRT debugging has been added long
ago[1], the leak checking is not usable for the test suite, because we
are no longer calling `xmlCleanupParser()` on RSHUTDOWN of
ext/libxml[2], and therefore a few bogus leaks are reported whenever
ext/libxml is unloaded.

We therefore ignore memory leaks for this case.  We introduce
`ZEND_IGNORE_LEAKS_BEGIN()` and `ZEND_IGNORE_LEAKS_END()` to keep
those ignores better readable, and also because these *might* be
useful for other leak checkers as well.

We also explicitly free the `zend_handlers_table` and the `p5s` to
avoid spurious leak reports.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=d756e1db2324c1f4ab6f9b52e329959ce6a02bc3>
[2] <http://git.php.net/?p=php-src.git;a=commit;h=8742276eb3905eb97a585417000c7b8df85006d4>