Test RTT switch reordering

[JoeTurki]

Description

a test to makes sure our SCTP loss-detection copes with a sudden RTT drop (high -> low) that causes packet reordering, without spuriously declaring losses.
It asserts that

1. No missing data: the client must receive all 40 echoes with correct payloads.
2. Currently disabled until rack is merged ( Add RACK to SCTP #390 ) no excessive fast retransmits both client and server report fast retransmit count <= 2, i.e., no widespread spurious loss due to the rtt shift.

Reference issue

@philipch07 asked me to help with this test. in discord dms :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.85%. Comparing base (c875185) to head (e6e1653).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #408      +/-   ##
==========================================
+ Coverage   81.77%   81.85%   +0.07%     
==========================================
  Files          51       51              
  Lines        4522     3439    -1083     
==========================================
- Hits         3698     2815     -883     
+ Misses        680      482     -198     
+ Partials      144      142       -2     

Flag	Coverage Δ
go	81.85% <ø> (+0.07%)	⬆️

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