Increase timeout of libio/tst-readline

Increase timeout from the default 20s to 100s. This test makes close to
20 million syscalls with distribution:

12327675 read
 4143204 lseek
  929475 close
  929471 openat
   92817 fstat
    1431 write
...

The default timeout assumes each can finish in 1us on average which
is not true on slow machines.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* libio/tst-readline.c (TIMEOUT): Define.

(cherry picked from commit ed643089)