Extract the code that is common in TestMessageResendAfterError,
TestMessageSendTimeout and TestMessageSendNotSupportedResponse to a
separate function.

Let prepareNetwork() make simmetric setup with two `ErrHost`s
with two `impl` networks to be sure we test `impl` instances on
both ends.

The test shows we have a problem with `MessagesSent` counter.

Fixes the bug with incrementing `MessagesSent` counter only in
`SendMessage()` method if `impl`. Now it works for `MessageSender`
too.

Added `listener network.Notifiee` to the `receiver` structure.
If a listener is specified then `prepareNetwork()` connects it
to the mock network it builds before making any connections.

Wait for all network streams are closed instead of just using a
timeout. The timeout of 5 s is still used as a deadline (it makes
the test to fail).

The `MessageSender` needs to be closed if we want all streams in
the network to be closed.