tests: Clear ignore_old_scan_res setting

tests: Clear ignore_old_scan_res setting

This parameter was used in some of the NFC test cases to make scanning
more robust in case of changing AP configuration. However, the parameter
was not cleared anywhere, so it could have been left in use for other
test cases as well. To get more consistent behavior, clear the value
back to its default between test cases.

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