tests: Make dbus_p2p_group_termination_by_go more robust

tests: Make dbus_p2p_group_termination_by_go more robust

Set peer_group_removed only if peer_group_added has already been set.
This fixes an issue where a propertiesChanged event triggered by an
earlier test case was able to get dbus_p2p_group_termination_by_go
terminated too early. This happened, e.g., with sequence
"dbus_p2p_two_groups dbus_p2p_group_termination_by_go".

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