executors: add version checks to self-tests

[kiritofeng]

Fixes #1108.

[codecov-commenter]

Codecov Report

Base: 81.81% // Head: 84.58% // Increases project coverage by +2.76% 🎉

Coverage data is based on head (3d8fc1a) compared to base (96b2fff).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1109      +/-   ##
==========================================
+ Coverage   81.81%   84.58%   +2.76%     
==========================================
  Files         139      139              
  Lines        5014     5013       -1     
==========================================
+ Hits         4102     4240     +138     
+ Misses        912      773     -139     

Impacted Files	Coverage Δ
dmoj/executors/C.py	100.00% <ø> (ø)
dmoj/executors/C11.py	100.00% <ø> (ø)
dmoj/executors/CPP11.py	100.00% <ø> (ø)
dmoj/executors/CPP14.py	100.00% <ø> (ø)
dmoj/executors/CPP17.py	100.00% <ø> (ø)
dmoj/executors/CPP20.py	100.00% <ø> (ø)
dmoj/executors/CLANGX.py	100.00% <100.00%> (ø)
dmoj/executors/CPP03.py	100.00% <100.00%> (ø)
dmoj/executors/PY2.py	100.00% <100.00%> (ø)
dmoj/executors/PY3.py	100.00% <100.00%> (ø)
... and 9 more

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[Xyene]

The fact that the C++03 executor is used as a "cpp_executor" of sorts is pretty yucky; we should just make a dedicated one at some point.

[Xyene]

LGTM, thanks!