test: degraded network support instructions

[matthiasn]

This PR adds instructions for running the Matrix integration test under adverse networking conditions, here with a delay of 1000 ms and a slow connection of 100 KB/s.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.89%. Comparing base (28d6d12) to head (1d01932).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1709   +/-   ##
=======================================
  Coverage   62.89%   62.89%           
=======================================
  Files         283      283           
  Lines       11482    11482           
=======================================
  Hits         7222     7222           
  Misses       4260     4260           

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