A picture is worth a thousand message digests

A picture is worth a thousand message digests

The bundled GD test suites makes heavy use of md5() to verify the result
of drawing operations. This leads to fragile tests (even a slight change
in a PNG header would cause failure, and of course there is the
possibility of collisions), and even worse, eventual test failures are
rather unrevealing.

Therefore we replace all md5() verification with a simplistic
test_image_equals_file(), which is basically a simplified port of libgd's
gdTestImageCompareToFile(), adapted to the needs of PHPTs.

In the long run better tests helpers should be introduced (see also
<http://news.php.net/php.internals/94081>), but for now this solution
is preferable over the former.

(cherry picked from commit 24f9e96792518ec2a75f26b1eb2471dd7694f2b7)