Test outputs starting with visitor and strategy

[gyermolenko]

• append_output.sh script

  • I didn't change overall script logic
  • updated output_marker according to idea discussed in [Infrastructure] Testing outputs #258
  • added a few notes on how script works for next readers

• updated strategy.py and visitor.py according to new scructure

def main():
    ...

if __name__ == "__main__":
   main()

OUTPUT = """
foo
bar
"""

• added test_outputs.py with generic test which should fit all scripts of the new structure

[codecov-io]

Codecov Report

Merging #262 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   97.65%   97.67%   +0.01%     
==========================================
  Files          61       61              
  Lines        2221     2238      +17     
==========================================
+ Hits         2169     2186      +17     
  Misses         52       52

Impacted Files	Coverage Δ
behavioral/visitor.py	100% <100%> (ø)	⬆️
behavioral/strategy.py	100% <100%> (ø)	⬆️
tests/test_outputs.py	100% <100%> (ø)

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[gyermolenko]

so I just found out from CI for py2 that contextlib.redirect_stdout is not available in py2 (only since 3.4).
Workarounds exist, but at the moment I don't see any problem performing test_outputs for py3 only.