feat[test]: add more example contracts

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

add more example contracts from third party sources to increase
compiler coverage, and our ability to detect compilation faults on
"in-the-wild" source code.

currently, the contracts are just tested for compilations, although in
the future we could add some integration / unit tests with them.

Description for the changelog

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.12%. Comparing base (95fcca7) to head (b5d58ca).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4500      +/-   ##
==========================================
+ Coverage   92.04%   92.12%   +0.07%     
==========================================
  Files         120      120              
  Lines       17364    17364              
  Branches     2942     2942              
==========================================
+ Hits        15983    15996      +13     
+ Misses        964      954      -10     
+ Partials      417      414       -3     

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