test: factor out swim from fakenet.c/.h files

SWIM unit tests contain special libraries for emulating event loop
and network: swim_test_ev and swim_test_transport. They provide
API similar to libev and to network part of libc, which internally
is implemented entirely in user-space and allows to simulate all
kinds of errors, any time durations, etc.

These test libraries are going to be re-used for Raft unit tests.
But for that it is necessary to detach them from all SWIM
dependencies.

--

This commit extracts all swim code to swim_test_transport.c. Now
this file is nothing but an implementation of swim_transport.h on
top of fakenet API.

Fakenet, in turn, does not depend on SWIM anymore, and can be
moved to its own library.

Part of #5303