Refactor Python tests with Pytest

[hcho3]

• Use Pytest features: parameterized tests, test fixture
• Turn on test coverage for the Python codebase
• Reformat tests to comply with pylint convention

[codecov-commenter]

Codecov Report

Merging #176 into master will decrease coverage by 4.80%.
The diff coverage is 73.33%.

@@             Coverage Diff              @@
##             master     #176      +/-   ##
============================================
- Coverage     85.20%   80.40%   -4.81%     
  Complexity       56       56              
============================================
  Files            58       81      +23     
  Lines          3644     4837    +1193     
  Branches         47       47              
============================================
+ Hits           3105     3889     +784     
- Misses          510      919     +409     
  Partials         29       29              

Impacted Files	Coverage Δ	Complexity Δ
python/treelite/core.py	64.91% <50.00%> (ø)	0.00 <0.00> (?)
python/treelite/gallery/sklearn/__init__.py	86.11% <66.66%> (ø)	0.00 <0.00> (?)
python/treelite/util.py	92.59% <66.66%> (ø)	0.00 <0.00> (?)
python/treelite/__init__.py	100.00% <100.00%> (ø)	0.00 <0.00> (?)
python/treelite/frontend.py	74.82% <100.00%> (ø)	0.00 <0.00> (?)
runtime/python/treelite_runtime/__init__.py	100.00% <100.00%> (ø)	0.00 <0.00> (?)
src/frontend/protobuf.cc	79.45% <0.00%> (-8.68%)	0.00% <0.00%> (ø%)
src/c_api/c_api.cc	76.94% <0.00%> (-8.48%)	0.00% <0.00%> (ø%)
...on/treelite/gallery/sklearn/rf_multi_classifier.py	0.00% <0.00%> (ø)	0.00% <0.00%> (?%)
python/treelite/contrib/gcc.py	100.00% <0.00%> (ø)	0.00% <0.00%> (?%)
... and 23 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4651255...b2424b7. Read the comment docs.