Add type signatures in test-network-mux

[coot]

Also consider splitting Test.Mux into submodules; it's a quite big module. Putting tests & architecture to run them in separate modules might be a good starting point.

[coot]

Some examples (a non exhaustive list) where the terms are quite big and are missing type signatures:

• https://github.com/input-output-hk/ouroboros-network/blob/master/network-mux/test/Test/Mux.hs#L435
• https://github.com/input-output-hk/ouroboros-network/blob/master/network-mux/test/Test/Mux.hs#L999
• https://github.com/input-output-hk/ouroboros-network/blob/master/network-mux/test/Test/Mux.hs#L1082