Add transaction testing framework

Please note: This is WIP.

Targets #339.

My approach: I have exactly one testing function which gets called in a loop,
check passes a `int` to it, which iteration is run atm. I use this `int`
as array index for the `inputs` array, which stores a input (which gets
deserialized) and an expeced output (which gets generated from the deserialized
message).

This can _only_ be done with "event" messages, as this is the only type
of message both the client and the "server" have to be able to talk.

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Thoughts on this:

1. An event from the client to the server is not the same as an event from
the server to the client. I guess we should redefine the API to look the same,
shouldn't we? I consider this as important, but _NOT_ before the end of the
project phase. We have more urgent things to do.

2. By now, this testing stuff has always buffers which are large enough.
Other tests (with smaller buffers) can be added easily, but I consider this as
secondary as well. An issue should be opened for this (with prio:low), right?

Probably a simple `buf[nbuf - 1] = 0;` would be enough