test: process IO swim test events before protocol's ones

Before that patch the swim test event loop worked like this: pop
a new event, set the global watch to its deadline, process the
event, repeat until the deadlines are the same. These events
usually generate IO events, which are processed next. But after
swim_quit() will be introduced, it is possible to insert new IO
events before protocol's events like round steps and ack checks.

Because of that it would be impossible to process new IO events
only, with timeout = 0, or with timeout > 0, but without changing
the global clock.

For example, a typical test would try to call swim_quit() on a
swim instance, and expect that it has sent all the quit messages
without delays immediately. But before this patch it would be
necessary to run at least one swim round to get to the IO
processing.

The patch splits protocol's events and IO events processing logic
into two functions and calls them explicitly in
swim_wait_timeout() - the main function to check something in the
swim tests.

Part of #3234