Add additional Entity unit tests

[Seth-Rothschild]

Add tests to test_entity

[codecov-io]

Codecov Report

Merging #136 into master will increase coverage by 0.75%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   89.98%   90.73%   +0.75%     
==========================================
  Files          76       76              
  Lines        7827     7824       -3     
==========================================
+ Hits         7043     7099      +56     
+ Misses        784      725      -59

Impacted Files	Coverage Δ
featuretools/tests/entityset_tests/test_entity.py	100% <100%> (ø)	⬆️
...computational_backends/calculate_feature_matrix.py	98.58% <0%> (-0.06%)	⬇️
featuretools/entityset/entityset.py	90.44% <0%> (+0.17%)	⬆️
featuretools/synthesis/deep_feature_synthesis.py	92.6% <0%> (+0.48%)	⬆️
featuretools/entityset/base_entity.py	79.16% <0%> (+2.38%)	⬆️
...turetools/computational_backends/pandas_backend.py	92.07% <0%> (+2.8%)	⬆️
featuretools/synthesis/encode_features.py	98.03% <0%> (+3.49%)	⬆️
featuretools/entityset/entity.py	90.29% <0%> (+10.91%)	⬆️

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[kmax12]

Looks good, merging