tests: Make active scans more robust

tests: Make active scans more robust

This makes testing under very heavy load or under extensive kernel
debugging options more robust by allowing number of test cases to scan
multiple times before giving up on active scans. The main reason for
many of the related test failures is in Probe Response frame from
hostapd not getting out quickly enough especially when multiple BSSes
are operating.

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