Core: Change the sleep time in dont_return.php from 100s to 30s

Half a minute is more than enough for the test to finish even in the slowest
browsers and in case of starving the PHP FPM process pool, one has to wait
for only half a minute to try again.

(cherry-picked from a10aa3aac5223c250327d00c500ceb9c7a18b7f9)