feat[test]: add hevm to get_contract harness

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

add `hevm` to the `get_contract` harness. if the `--hevm` flag
is set when running the test suite, the source code argument to
`get_contract()` will additionally be compiled and equivalence-checked
between venom and non-venom bytecode, using `hevm equivalence`.

individual tests can be marked (as is done in this commit) for
inclusion in the hevm CI job, with `pytest.mark.hevm`, and then any
calls to `get_contract` will run the equivalence checker as described
above.

the idea behind this commit is to makes it easy to increase hevm
equivalence coverage in our test suite, simply by manually annotating
individual tests with `pytest.mark.hevm`.

Description for the changelog

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.16%. Comparing base (fbcf7d2) to head (29b7baf).
Report is 86 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4499   +/-   ##
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  Coverage   92.16%   92.16%           
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  Files         122      122           
  Lines       17471    17471           
  Branches     2959     2959           
=======================================
  Hits        16103    16103           
  Misses        957      957           
  Partials      411      411           

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