tests: Make ap_wps_er_add_enrollee less likely to fail

tests: Make ap_wps_er_add_enrollee less likely to fail

WPS-ER-AP-REMOVE event from the ER is sent before HTTP UNSUBSCRIBE has
been completed. As such, it was possible for the following scan
validation step to be started before the AP has had a chance to react to
the ER status change. Makes this less likely to fail by waiting 200 ms
before starting the last scan.

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