Fix two sum unit_test challenge to match the solution notebook (donnemartin#176)

Author: ammarnajjar

arrays_strings/two_sum/two_sum_challenge.ipynb

@@ -122,8 +122,12 @@
     "\n",
     "    def test_two_sum(self):\n",
     "        solution = Solution()\n",
-    "        assert_raises(TypeError, solution.two_sum, None)\n",
-    "        assert_equal(solution.two_sum(0), 0)\n",
+    "        assert_raises(TypeError, solution.two_sum, None, None)\n",
+    "        assert_raises(ValueError, solution.two_sum, [], 0)\n",
+    "        target = 7\n",
+    "        nums = [1, 3, 2, -7, 5]\n",
+    "        expected = [2, 4]\n",
+    "        assert_equal(solution.two_sum(nums, target), expected)\n",
     "        print('Success: test_two_sum')\n",
     "\n",
     "\n",