tests: Fix ap_track_taxonomy to clear country code setting

tests: Fix ap_track_taxonomy to clear country code setting

This test case could have ended with the station devices still in US
regulatory domain and that could make a following test case fail in some
sequences. For example, "ap_track_taxonomy ibss_5ghz" sequence made
ibss_5ghz fail to see the regdom change event since there was not one
due to the US country code already being in use at the beginning of the
test case. Fix this by clearing the country code at the end of
ap_track_taxonomy.

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