tests: Add sleep to ensure wlantest processed packets

tests: Add sleep to ensure wlantest processed packets

At least with EEVDF it appears to occasionally happen that wlantest has
not processed all packets but does respond to the command to query the
BSS counter. Fix this by inserting a sleep.

An alternative approach could be to force command handling into a
timeout or introducing a priority concept to eloop.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>