test: add mocked Sock that can read/write custom data and/or CNetMessages #30205

1. 
   vasild commented at 3:49 pm on May 30, 2024: contributor

   Put the generic parts from StaticContentsSock into a separate class ZeroSock so that they can be reused in other mocked Sock implementations.

   Add a new DynSock whose Recv() and Send() methods can be controlled after the object is created. To achieve that, the caller/creator of DynSock provides to its constructor two pipes (FIFOs) - recv-pipe and send-pipe. Whatever data is written to recv-pipe is later received by DynSock::Recv() method and whatever data is written to the socket using DynSock::Send() can later be found in the send-pipe. For convenience there are also two methods to send and receive CNetMessages.

   ---

   This is used in #26812 (first two commits from that PR). Extracting as a separate PR suggested here: #30043 (review).

2. test: put the generic parts from StaticContentsSock into a separate class

   This allows reusing them in other mocked implementations.
   
   Also move the implementation (method definitions) to
   `test/util/net.cpp` to make the header `test/util/net.h` easier to follow.

   e180405d99
3. test: add a mocked Sock that allows inspecting what has been Send() to it

   And also allows gradually providing the data to be returned by `Recv()`
   and sending and receiving net messages (`CNetMessage`).

   06b21ab6cb
4. 
   DrahtBot commented at 3:49 pm on May 30, 2024: contributor

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5. DrahtBot added the label Tests on May 30, 2024
6. 
   Sjors commented at 4:31 pm on May 30, 2024: member

   This seems very useful. I’ll try to use it for the (currently very brittle) Sv2Transport tests in #29432, and review it along the way.

Contributors
vasild DrahtBot Sjors

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Tests