test: generalize SWIM fake descriptor filters

At this moment SWIM test harness implements its own fake file
descriptor table, which is used unawares by the real SWIM code.
Each fake fd has send and recv queues, can delay and drop
packets with a certain probability. But it is going to be not
enough for new tests.

It is wanted to be able to drop packets with a specified content,
from and to a specified direction. For that the patch implements
a filtering mechanism. Each fake fd now has a list of filters,
applied one by one to each packet. If at least on filter wants to
drop a packet, then it is dropped. The filters know packet
content and direction: outgoing or incomming.

Now only one filter exists - drop rate. It existed even before
the patch, but now it is ported on the new API.

Part of #3234