Make gh7875.phpt more resilient

Apparently, on Cirrus CI FreeBSD chmodding a file to 0444 doesn't make
it readonly.  So in this case, we skip the test.

[1] <#7975>

ext/standard/tests/mail/gh7875.phpt

@@ -1,5 +1,16 @@
 --TEST--
 GH-7875 (mails are sent even if failure to log throws exception)
+--SKIPIF--
+<?php
+$filename = __DIR__ . "/gh7875.mail.log";
+touch($filename);
+chmod($filename, 0444);
+clearstatcache();
+$is_writable = is_writable($filename);
+chmod($filename, 0644);
+unlink($filename);
+if ($is_writable) die("skip cannot make file read-only");
+?>
 --INI--
 sendmail_path={MAIL:{PWD}/gh7875.mail.out}
 mail.log={PWD}/gh7875.mail.log