tests: Check operating channel parameter in obss_scan*

tests: Check operating channel parameter in obss_scan*

It looks like these test cases can fail with the new "Undefined
secondary channel: drop OBSS scan results" case. Add more checks to
determine if something is wrong with the connection.

In addition, force clearing of the cfg80211 scan cache on the main AP
interface so that a scan result from a previous test case cannot prevent
40 MHz channel bandwidth from being used. This could apparently happen
in the following test case sequence:
ap_ht40_scan_conflict obss_scan

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