add integration tests using faulty packet conns

[marten-seemann]

Fixes #2659. Depends on #2656.

This would have caught #2655. @ltucker, is it realistic to assume that if a connection fails, both ReadFrom and WriteTo will return an error?

[codecov]

Codecov Report

Merging #2663 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2663   +/-   ##
=======================================
  Coverage   86.62%   86.62%           
=======================================
  Files         122      122           
  Lines        9749     9749           
=======================================
  Hits         8445     8445           
  Misses        971      971           
  Partials      333      333           

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[ltucker]

@marten-seemann oh sorry missed this, I think this is usually the case relatively quickly in practice... I don't think golang technically claims anything about this property of the interface. I can imagine these error-ing at slightly different times due to differences in deadlines or something, but may not be worth the fidgety-ness / buy much.