Fix test expectation

The `W32_SM_SENDMAIL_FROM_MALFORMED` error message will only be shown
if the `$additional_headers` parameter is passed to `mail`, which is
not the case for this test.  Instead we have to expect `BAD_MSG_RPATH`.

ext/standard/tests/mail/mail_variation_alt3-win32.phpt

@@ -87,5 +87,5 @@ imap_close($imap_stream, CL_EXPUNGE);
 --EXPECTF--
 *** Testing mail() : basic functionality ***
 
-Warning: mail(): "sendmail_from" not set in php.ini or custom "From:" header missing in %s on line %d
+Warning: mail(): Bad Message Return Path in %s on line %d
 TEST COMPLETED : Unable to send test email