Fix flaky extra mail header (phpGH-8086)

As documented in [RunningTests], tests have to be written to be
independent of any php.ini file.

As the tests are based on counting the line terminators, the
mail.add_x_header php.ini setting has to be off.

Fix is to set `mail.add_x_header` to `off`.

[RunningTests]: docs/source/miscellaneous/running-tests.rst
fix-up-of: phpGH-8086

Author: hakre

ext/standard/tests/mail/gh8086.phpt

@@ -3,6 +3,7 @@ GH-8086 (Mail() function not working correctly in PHP 8.x)
 --INI--
 sendmail_path={MAIL:gh8086.out}
 mail.mixed_lf_and_crlf=on
+mail.add_x_header=off
 --FILE--
 <?php
 var_dump(mail('[email protected]', 'Test Subject', 'A Message', 'KHeaders'));