Add a test to prepare for integer default_left

[hcho3]

Respond to dmlc/xgboost#7556, which changes the type of default_left field in XGBoost models to integer array (the field used to be a boolean array).

It turns out that we don't need to change the parser at all. This PR simply adds a unit test.

cc @trivialfis

[codecov]

Codecov Report

Merging #337 (1835f92) into mainline (09521bc) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##             mainline     #337      +/-   ##
==============================================
+ Coverage       84.86%   84.92%   +0.05%     
  Complexity         42       42              
==============================================
  Files             108      108              
  Lines            8420     8455      +35     
  Branches          470      471       +1     
==============================================
+ Hits             7146     7180      +34     
- Misses           1251     1252       +1     
  Partials           23       23              

Impacted Files	Coverage Δ
src/frontend/xgboost/xgboost_json.h	89.81% <100.00%> (+0.39%)	⬆️
tests/cpp/test_frontend.cc	94.02% <100.00%> (+0.60%)	⬆️
src/annotator.cc	91.07% <0.00%> (-1.20%)	⬇️
src/threading_utils/parallel_for.h	100.00% <0.00%> (+2.38%)	⬆️

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