tests: Make *_key_lifetime_in_memory more robust

tests: Make *_key_lifetime_in_memory more robust

It was possible for the GTK-found-in-memory case to be triggered due to
a retransmission of EAPOL-Key msg 3/4 especially when running test cases
under heavy load (i.e., timeout on hostapd due to not receiving the 4/4
response quickly enough). Make this false failure report less likely by
waiting a bit longer after the connection has been completed before
fetching the process memory.

Signed-off-by: Jouni Malinen <j@w1.fi>