Make imagebmp_basic.phpt more resilient (GH-17265)

Checking for the exact output of most image formats is brittle; even an
otherwise change to some header field causes the output to change, even
if the image would be visually identical.

Checking for an (MD5) hash is even worse, since if the tests fails, we
have no clue for what reason.

Thus we compare the generated image against an pre-generated PNG, using
a test helper which will output a simple image diff in case of test
failure.

ext/gd/tests/imagebmp_basic.phpt

@@ -14,10 +14,8 @@ imagecolorallocate($im, 0, 0, 0);
 $white = imagecolorallocate($im, 255, 255, 255);
 imageline($im, 10,10, 89,89, $white);
 
-// write the md5 hash of its BMP representation
-ob_start();
-imagebmp($im);
-echo md5(ob_get_clean());
+require __DIR__ . "/func.inc";
+test_image_equals_file(__DIR__ . "/imagebmp_basic.png", $im);
 ?>
 --EXPECT--
-d49124076771822b09fa72e168c0de56
+The images are equal.