feat: add support for return-broken-stream-after-YK

[cojenco]

Fixes #173

This introduces support for return-broken-stream-after-YK of downloaded data in the Retry Test API.
To better test complex download scenarios in the retry conformance tests, we've discussed to test larger files and add support for "after-YK" instead of "after-YB"

• Tests pass
• Appropriate changes to README are included in PR

[codecov]

Codecov Report

Merging #276 (f8ed389) into main (ebf4d23) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #276   +/-   ##
=======================================
  Coverage   97.61%   97.62%           
=======================================
  Files          48       48           
  Lines        7598     7629   +31     
=======================================
+ Hits         7417     7448   +31     
  Misses        181      181           

Flag	Coverage Δ
unittests	97.62% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
testbench/database.py	100.00% <ø> (ø)
testbench/common.py	98.67% <100.00%> (+0.02%)	⬆️
tests/test_testbench_retry.py	99.47% <100.00%> (+0.06%)	⬆️

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[tritone]

Couple little comments, LGTM overall. Thanks!