tcp: add netsim test

[Dirbaio]

This test simulates a network with a given latency and packet loss, and measures the
throughput between two virtual smoltcp instances. It seems to match what's expected. For example given a 1024b buffer the expected throughput is 1024b/100ms=10240b/s (at zero loss). and it checks out. Throughput goes down exponentially as loss increases.

Depends on #1025 for fixes to actually make tcp work and not get stuck. For example lack of zero window probes, and retransmits were broken in some edge cases.

I'm not sure how to actually use the test results. Options:

• (what this PR implements) snapshot the throughputs into a text file, check it into git. Every time someone touches something that changes the throughput they'll have to do INSTA_UPDATE=always cargo test to update the snapshot and get the test to pass, and in the PR diff we'll be able to review the effect on the throughput.
• Sameas above, but reinvent it ourselves instead of adding insta to save a dependency.
• Just assert throughput is higher than a number we pick as acceptable throughput. Simpler but won't catch small variations.
• Make some bot like rustc's perf that runs it and posts the results in a PR comment and allows diffing them. Probably too overengineered.

Future work:

• Simulate a network max throughput. Will be useful to test the congestion controllers.
• Test usage patterns other than one-way throughput. For example non-pipelined request+response. Will be useful to test the impact of delayed ack, nagle, etc.

[codecov]

Codecov Report

Attention: Patch coverage is 99.01768% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.17%. Comparing base (cce735e) to head (512aecb).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/socket/tcp.rs	99.01%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1026      +/-   ##
==========================================
+ Coverage   80.84%   81.17%   +0.33%     
==========================================
  Files          81       81              
  Lines       28485    28954     +469     
==========================================
+ Hits        23029    23504     +475     
+ Misses       5456     5450       -6     

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