tests: memcached/test_memcached.py: sleep more when testing expiration/flush
tests: memcached/test_memcached.py: sleep more when testing expiration/flush

Increase the sleep time by one second in expiration/flush
tests in order to take into an account a fact that a command
may come on a second boundary and thus the result of the
expiration/flushing may be seen a second later than the exact
requested amount of seconds from "now".

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

scylladb/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,sjperkins/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,sjperkins/seastar,scylladb/seastar,sjperkins/seastar,kjniemi/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,kjniemi/seastar,kjniemi/seastar,syuu1228/seastar