Most of our IP stacks don't allow removal of interfaces so
interface destructor can not reliably clean up. Therefore we
cannot rely its behaviours in test case.
Instead run interface->disconnect() in case interface was already
created.
The tests try to:
* scan for available networks and check whether specified networks
are present in the results.
* connect to and disconnect from the specified network.
* repeats the scan tests while connected to a network.
* connect to a network and perform simple HTTP query.