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 moves all fake network code to separate files -
fakenet.c/.h, which are now easy to relocate to a new library.

These files still contain some swim methods, which are moved to
their own file in a separate commit. Because their file will be
swim_test_transport.c. But if they would be moved there in the
same commit, git would treat it like everything *except* the swim
methods was moved to fakenet.c/.h because of names clash. That
would destroy git history. So the swim code movement is split in 2
commits.

Part of #5303