Add a regression test for bitcoin#21407.

The test creates a socket that, upon read, returns some data, but never
the expected terminator `\n`, injects that socket into the I2P code and
expects `i2p::sam::Session::Connect()` to fail, printing a specific
error message to the log.

7059e6d test: add a test to ensure RecvUntilTerminator() limit works (Vasil Dimov)
80a5a8e i2p: limit the size of incoming messages (Vasil Dimov)

Pull request description:

  Put a limit on the amount of data `Sock::RecvUntilTerminator()` can read
  if no terminator is received.

  In the case of I2P this avoids a runaway (or malicious) I2P proxy
  sending us tons of data without a terminator before a timeout is
  triggered.

ACKs for top commit:
  laanwj:
    Re-ACK 7059e6d

Tree-SHA512: 21f3f3468c765c726cdc877eae847cdb4dbe225d94c5bd1849bd752c9761fac881c554f16ea7a685ad40312159d554e126c481e21c5fd83a6d947060b920373d

40316a3 test: add I2P test for a runaway SAM proxy (Vasil Dimov)
2d8ac77 fuzz: add tests for the I2P Session public interface (Vasil Dimov)
9947e44 i2p: use pointers to Sock to accommodate mocking (Vasil Dimov)
82d360b net: change ConnectSocketDirectly() to take a Sock argument (Vasil Dimov)
b586110 net: add connect() and getsockopt() wrappers to Sock (Vasil Dimov)
5a887d4 fuzz: avoid FuzzedSock::Recv() repeated errors with EAGAIN (Vasil Dimov)
3088f83 fuzz: extend FuzzedSock::Recv() to support MSG_PEEK (Vasil Dimov)
9b05c49 fuzz: implement unimplemented FuzzedSock methods (Vasil Dimov)

Pull request description:

  Change the networking code and the I2P code to be fully mockable and use `FuzzedSocket` to fuzz the I2P methods `Listen()`, `Accept()` and `Connect()`.

  Add a mocked `Sock` implementation that returns a predefined data on reads and use it for a regression unit test for the bug fixed in bitcoin/bitcoin#21407.

ACKs for top commit:
  practicalswift:
    Tested ACK 40316a3
  MarcoFalke:
    Concept ACK 40316a3
  jonatack:
    re-ACK 40316a3 reviewed `git range-diff 01bb3af 23c861d 40316a3` and the new unit test commit, debug built, ran unit tests, ran bitcoind with an I2P service and network operation with seven I2P peers (2 in, 5 out) is looking nominal
  laanwj:
    Code review ACK 40316a3

Tree-SHA512: 7fc4f129849e16e0c7e16662d9f4d35dfcc369bb31450ee369a2b97bdca95285533bee7787983e881e5a3d248f912afb42b4a2299d5860ace7129b0b19623cc8

Summary:
> net: add connect() and getsockopt() wrappers to Sock
>
> Extend the `Sock` class with wrappers to `connect()` and `getsockopt()`.
>
> This will make it possible to mock code which uses those.
bitcoin/bitcoin@b586110

> net: change ConnectSocketDirectly() to take a Sock argument
>
> Change `ConnectSocketDirectly()` to take a `Sock` argument instead of a
> bare `SOCKET`. With this, use the `Sock`'s (possibly mocked) methods
> `Connect()`, `Wait()` and `GetSockOpt()` instead of calling the OS
> functions directly.
bitcoin/bitcoin@82d360b

> i2p: use pointers to Sock to accommodate mocking
>
> Change the types of `i2p::Connection::sock` and
> `i2p::sam::Session::m_control_sock` from `Sock` to
> `std::unique_ptr<Sock>`.
>
> Using pointers would allow us to sneak `FuzzedSock` instead of `Sock`
> and have the methods of the former called.
>
> After this change a test only needs to replace `CreateSock()` with
> a function that returns `FuzzedSock`.
bitcoin/bitcoin@9947e44

> test: add I2P test for a runaway SAM proxy
>
> Add a regression test for bitcoin/bitcoin#21407.
>
> The test creates a socket that, upon read, returns some data, but never
> the expected terminator `\n`, injects that socket into the I2P code and
> expects `i2p::sam::Session::Connect()` to fail, printing a specific
> error message to the log.
bitcoin/bitcoin@40316a3

This is a partial backport of [[bitcoin/bitcoin#21387 | core#21387]], without the fuzzer changes.
Depends on D11037

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11042