tests: Fix wifi_display_persistent_group event checks

tests: Fix wifi_display_persistent_group event checks

The final invitation case in this test was verified incorrectly. The GO
was already operatign in that case, so there was not supposed to be a
new P2P-GROUP-STARTED message from GO. That happened to be show up in
the pending event messages from the last instance, but that was just by
accident and any additional dump_monitor() operation added here would
cause this test to fail. Fix this by handling the final invitation
separately and verifying that only the client side indicates
P2P-GROUP-STARTED.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>