tests: Make PMKSA caching tests more robust

tests: Make PMKSA caching tests more robust

When the STA is forced to disconnect immediately after completion of
4-way handshake, there is a race condition on the AP side between the
reception of EAPOL-Key msg 4/4 and the following Deauthentication frame.
It is possible for the deauthentication notification to be processed
first since that message uses different path from kernel to user space.

If hostapd does not receive EAPOL-Key msg 4/4 prior to deauthentication,
no PMKSA cache entry is added. This race condition was making the test
cases expecting PMKSA caching to work to fail every now and then. Avoid
this issue by waiting for AP-STA-CONNECTED event from hostapd. This
makes sure the PMKSA cache entry gets added on the AP side.

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