[WIP] Issue#98: refactor test suite

[gyermolenko]

Disclaimer: work in progress. I need initial review to proceed.

Main points:

• split tests by topics (instantiation, contents, comparisons)
• create parametrized fixtures - all 4 types of instances ([MultiDict, CIMultiDict, MultiDictProxy, CIMultiDictProxy]) are pre-created and used for tests which concern not instantiation of those classes themselves (unfortunately, pytest so far [doesn't support fixtures in pytest.mark.parametrize] )(https://github.com/pytest-dev/pytest/issues/349)
• ~~use autouse=True class level hack (since we can't use __init__) to be able to use self.d instead of long fixture names.~~

Since my goal is restructuring - I didn't touch asserts themselves. I think that such comments can be addressed after the fact.

[codecov[bot]]

Codecov Report

Merging #294 into master will decrease coverage by 0.1%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   93.48%   93.37%   -0.11%     
==========================================
  Files          10        5       -5     
  Lines         936      468     -468     
==========================================
- Hits          875      437     -438     
+ Misses         61       31      -30

Impacted Files	Coverage Δ
multidict/_multidict_py.py
multidict/__init__.py
multidict/_multidict_base.py
multidict/_compat.py
multidict/_abc.py

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[gyermolenko]

@asvetlov sorry to bother, just a ping Since this WIP needs your preliminary review before I proceed (or abort) with it.

[asvetlov]

Will do. Sorry for a pause

[gyermolenko]

hey, @asvetlov @webknjaz I've updated this PR and my first comment. Please review and leave your comments :)

[gyermolenko]

hey @asvetlov , just a friendly ping. It would be nice to have your comments on updated PR

[webknjaz]

If this is still relevant after #915, please recreate the PR with those fixtures.