MAINT: rename build_fn to model in tests

[adriangb]

Closes #98

[codecov-io]

Codecov Report

Merging #98 into master will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##            master      #98      +/-   ##
===========================================
- Coverage   100.00%   99.81%   -0.19%     
===========================================
  Files            3        3              
  Lines          526      528       +2     
===========================================
+ Hits           526      527       +1     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
scikeras/wrappers.py	99.74% <100.00%> (-0.26%)	⬇️

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