Concatenate Strings

Write a function concatenate_strings that receives two strings and returns a new one containing both strings concatenated. Example:

concatenate_strings('Hello', 'World')  # HelloWorld
concatenate_strings('Hello', '')  # Hello
import unittest


class ConcatenateStringTestCase(unittest.TestCase):

    def test_with_multiple_strings(self):
        self.assertEqual(concatenate_strings('abc', 'def'), 'abcdef')

    def test_with_empty_string(self):
        self.assertEqual(concatenate_strings('abc', ''), 'abc')