[CI] Run tests on Windows

[hcho3]

I ended up finding some issue when the unit tests were run on the Windows target.

• More exhaustive method to locate vcvarsall.bat
• Do not use GCC on Windows
• Do not produce empty array literals, as they are forbidden by MSVC
• Per TestLightGBMIntegration::test_sparse_categorical_model test fails with MSVC and Apple Clang tl2cgen#13, Skip test_sparse_categorical_model() test on Windows for now

[codecov-io]

Codecov Report

Merging #101 into master will decrease coverage by 0.23%.
The diff coverage is 71.01%.

@@             Coverage Diff              @@
##             master     dmlc/treelite#101      +/-   ##
============================================
- Coverage     86.15%   85.91%   -0.24%     
  Complexity       49       49              
============================================
  Files            81       81              
  Lines          5741     5772      +31     
  Branches         37       37              
============================================
+ Hits           4946     4959      +13     
- Misses          768      786      +18     
  Partials         27       27

Impacted Files	Coverage Δ	Complexity Δ
python/treelite/frontend.py	72.56% <100%> (+0.09%)	0 <0> (ø)	⬇️
tests/python/test_basic.py	100% <100%> (ø)	0 <0> (ø)	⬇️
tests/python/test_lightgbm_integration.py	97.43% <100%> (ø)	0 <0> (ø)	⬇️
tests/python/test_model_builder.py	100% <100%> (ø)	0 <0> (ø)	⬇️
python/treelite/common/util.py	97.05% <100%> (+15.24%)	0 <0> (ø)	⬇️
python/treelite/contrib/__init__.py	65.97% <50%> (-2.26%)	0 <0> (ø)
src/compiler/ast_native.cc	90.56% <68.18%> (-5.95%)	0 <0> (ø)
runtime/native/src/thread_pool/spsc_queue.h	94.44% <0%> (+5.55%)	0% <0%> (ø)	⬇️

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[hcho3]

There is a bug in subprocess.Popen() on Windows: https://bugs.python.org/issue37380. The bug is causing crash in export_lib(). This bug was fixed in latest versions of 3.7 and 3.8 (dev) (python/cpython#14360). I will try to update Python in the test runner and see if crash is fixed.

[hcho3]

Using latest Python doesn't make any difference. I'd have to conclude that Azure Pipelines doesn't let a process to write to a temporary directory underC:\\Users\\VssAdministrator\\AppData\\Local\\Temp.

[hcho3]

Note to myself: the LETOR model takes inordinately long time to compile on Windows (with MSVC), taking more than an hour (the job gets killed by Azure Pipelines). The same model takes < 5 minutes on Linux however. I suspect it has to do with IO performance difference between Linux and Windows.