tests: Make dbus_p2p_group_idle_timeout more robust

tests: Make dbus_p2p_group_idle_timeout more robust

This test case was failing if a PropertiesChanged signal for P2P peer
gets delivered from a previous test case. Avoid that by waiting for the
new group to be formed before processing any PropertiesChanged signals.

This failure was triggered by the following test case sequence:
dbus_p2p_two_groups dbus_p2p_group_idle_timeout

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