libgoal: output debug info on failure to stderr

[algorandskiy]

Summary

The test failed again and again there is no debug info, probably because either test logger gone or some flushing issue.
Using fmt.Fprintf(os.Stderr, ...) should take take about both about possibilities.

Test Plan

This is a test harness change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.91%. Comparing base (4caf2e4) to head (4b2d300).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6027      +/-   ##
==========================================
+ Coverage   55.86%   55.91%   +0.04%     
==========================================
  Files         482      482              
  Lines       68457    68457              
==========================================
+ Hits        38245    38278      +33     
+ Misses      27608    27581      -27     
+ Partials     2604     2598       -6     

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