test: Add a way to mock peers for SignalProxy-related test cases

Introduce a new mock class MockedPeer that allows for setting
expectations for dispatched Quassel protocol messages. Also provide
matchers to express such expectations in a straightforward way.

MockedPeer for now only supports SignalProxy messages; support for
authentication messages can be added later if needed. Note that
expectations can be set on the level of protocol messages, but things
like a testing a particular serialization, the use of network sockets,
or other lower-level implementation details are not supported, nor is
MockedPeer intended for such use cases (we'd expect specific tests for
specific peer types).