Provide utility function to verify DataFrames

[yunfengzhou-hub]

Currently, some tests in feathub verify a pandas DataFrame as follows.

self.assertTrue(expected_df.equals(result_df)

Such implementation cannot work in certain cases. For example, if an element in the two DataFrames is a dict whose order of entries is different, they are supposed to pass the test, while the current implementation fails in this case.

Thus we should use a better way to verify DataFrames.