add a send stream test that randomly acknowledges and loses data

[marten-seemann]

In an attempt to find the race condition of #2533 I wrote this test. Turns out that the race condition is not caused by handling of lost frames in the stream.

[codecov-io]

Codecov Report

Merging #2535 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2535      +/-   ##
==========================================
+ Coverage   86.12%   86.16%   +0.03%     
==========================================
  Files         122      122              
  Lines        9542     9542              
==========================================
+ Hits         8218     8221       +3     
+ Misses        985      983       -2     
+ Partials      339      338       -1     

Impacted Files	Coverage Δ
send_stream.go	93.36% <0.00%> (+0.37%)	⬆️
server.go	83.59% <0.00%> (+0.61%)	⬆️

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