tests: Try to clear scan results after regulatory domain changes

tests: Try to clear scan results after regulatory domain changes

It was possible for regulatory domain changes to "leak" into following
test cases in number of cases where the cfg80211 BSS table remained
after regulatory domain had been restored to world roaming. Try to make
this less likely to occur by explicitly clearing BSS table at the end of
test cases that use different regulatory domain. This makes P2P test
cases that verify channel selection based on world roaming rules more
robust.

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