Update tests in test_deep_feature_synthesis.py

[thehomebrewnerd]

Update tests in test_deep_feature_synthesis.py

The tests in tests/synthesis/test_deep_feature_synthesis.py appear to be testing various aspects of the DeepFeatureSynthesis object. There are a few tests in this file that generate a list of features using a call to ft.dfs. These tests could be updated to use a call to DeepFeatureSynthesis.build_features() instead. This would better align with the other tests in the file, and would improve test clarity as they are not testing any other aspects of the dfs method.