tests: Fix ap_vht160_no_dfs false negative by using common finalizer

tests: Fix ap_vht160_no_dfs false negative by using common finalizer

ap_vht160_no_dfs fails with this message:

---------------
wlan0: Country code not reset back to 00: is ZA
wlan0: Country code cleared back to 00
---------------

hostap commit 91b6eba7732354ed3dfe0aa9715dc4c0746e3336
'Move MAC address randomization enable/disable to helper functions'.

wireless-testing commit 66c112cbd1d44d05322bb4eef908c82a68adbb5e
tag: wt-2019-06-26.

This patch fixes the issue.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>