[RFC] tests: add tests for HS 2.0 frame filtering

[Johannes Berg]

A few notes...

First of all, of course this is conditional on the kernel patches being
accepted.

> +    procfile = '/proc/sys/net/ipv4/conf/%s/drop_unicast_in_l2_multicast' % dev.ifname

Secondly, with all this proc-file access, this is clearly a very
low-level test.

We'll need some logic in wpa_supplicant to set these knobs
appropriately, and once we have that we should have another set of tests
that checks that when we connect to a suitable HS2 AP they get set
correctly (and the filtering works as expected). Some of the test code
from here could be lifted for that, but the changes to the sysctl knobs
clearly should come from wpa_supplicant instead of the test code then.

johannes