add tests for feature importance on regression

[ExpandingMan]

addressed #37

[codecov-commenter]

Codecov Report

Merging #38 (9a3d40d) into master (bc3ecef) will increase coverage by 0.79%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   53.96%   54.76%   +0.79%     
==========================================
  Files           1        1              
  Lines         126      126              
==========================================
+ Hits           68       69       +1     
+ Misses         58       57       -1     

Impacted Files	Coverage Δ
src/MLJXGBoostInterface.jl	54.76% <0.00%> (+0.79%)	⬆️

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[ablaom]

Thanks.

This PR adds a test for XGBoostCount
We still need:

• feature importance test for XGBoostRegressor
• serialize/deserialize test for XGBoostCount
• serialize/deserialize test for XGBoostRegressor

[ExpandingMan]

Ok, I'll add them when I get a chance.

[ExpandingMan]

I messed up my branches, so I'm closing this, it'll be replaced in a moment.