Add unittest for discriminator

[koxudaxi]

Closes: #1474

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (1d5b0bd) 100.00%.
Report is 695 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1488     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        32     +21     
  Lines         1020      3581   +2561     
  Branches       201       839    +638     
===========================================
+ Hits          1020      3581   +2561     

Flag	Coverage Δ
unittests	99.63% <99.58%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/imports.py	100.00% <100.00%> (ø)
... and 22 more

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