Adapt multidict C ext test implementation

[bdraco]

What do these changes do?

Adds conftest facade for handling the C ext

Are there changes in behavior for the user?

no

Related issue number

#18

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (560c078) to head (10fcfdd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   85.98%   87.01%   +1.02%     
==========================================
  Files          20       21       +1     
  Lines         678      724      +46     
  Branches       50       58       +8     
==========================================
+ Hits          583      630      +47     
+ Misses         68       67       -1     
  Partials       27       27              

Flag	Coverage Δ
CI-GHA	87.01% <100.00%> (+1.32%)	⬆️
MyPy	67.79% <49.72%> (-1.14%)	⬇️
OS-Linux	98.52% <98.22%> (-0.21%)	⬇️
OS-Windows	93.07% <93.49%> (+0.21%)	⬆️
OS-macOS	93.75% <93.49%> (-2.88%)	⬇️
Py-3.10.11	92.64% <92.30%> (-3.56%)	⬇️
Py-3.10.15	95.58% <97.04%> (-2.31%)	⬇️
Py-3.11.10	95.58% <97.04%> (-2.31%)	⬇️
Py-3.11.9	92.64% <92.30%> (-3.56%)	⬇️
Py-3.12.6	95.52% <97.04%> (-1.96%)	⬇️
Py-3.13.0-rc.3	95.52% <97.04%> (-1.96%)	⬇️
Py-3.8.10	92.98% <92.85%> (-3.22%)	⬇️
Py-3.8.18	95.94% <97.61%> (-1.95%)	⬇️
Py-3.9.13	92.61% <92.26%> (-3.59%)	⬇️
Py-3.9.20	95.57% <97.02%> (-2.32%)	⬇️
Py-pypy7.3.11	95.65% <97.02%> (+1.26%)	⬆️
Py-pypy7.3.16	95.21% <96.42%> (+0.82%)	⬆️
Py-pypy7.3.17	95.23% <96.44%> (+0.85%)	⬆️
VM-macos-latest	93.75% <93.49%> (-2.88%)	⬇️
VM-ubuntu-latest	98.52% <98.22%> (-0.21%)	⬇️
VM-windows-latest	93.07% <93.49%> (+0.21%)	⬆️
pytest	98.40% <98.22%> (-0.16%)	⬇️

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