test: fix sporadic box/net.box failure

From time to time box/net.box test fails like this:

  Test failed! Result content mismatch:
  --- box/net.box.result	Sat Aug 25 18:41:35 2018
  +++ box/net.box.reject	Sat Aug 25 18:49:17 2018
  @@ -2150,7 +2150,7 @@
   ...
   disconnected -- false
   ---
  -- false
  +- true
   ...
   ch2:put(true)
   ---

The 'disconnected' variable is changed from false to true by
box.session.on_disconnect trigger. If there happens to be a
dangling connection that wasn't explicitly closed, it might
occur that the Lua garbage collector closes it, which will
result in spurious trigger execution and test failure.

Fix this by collecting all dangling connections before installing
the trigger.