test: create isolated ev_loop for swim unit tests

The SWIM unit tests code with the fake events and time does lots
of forbidden things: it manually invokes pending watcher
callbacks; manages global time without a kernel; puts not
existing descriptors into the loop. These foul blows open the
gates to the full control over IO events, descriptors, virtual
time. Hundreds of virtual seconds pass in milliseconds in
reality, it makes SWIM unit tests fast despite complex logic.

All these actions does not affect the loop until yield. On yield
a scheduler fiber wakes up and

    1) infinitely generates EV_READ on not existing descriptors
       because a kernel considers them closed;

    2) manual pending callbacks invocation asserts, because it is
       not allowed for non-scheduler fibers.

To avoid these problems a new isolated loop is created, not
visible for the scheduler. Here the fake events library can rack
and ruin whatever it wants.

Needed for #4250