Added mul function.

tests/test_main.py

@@ -1,5 +1,5 @@
 import pytest
-from poetry_template.main import sum
+from poetry_template.main import sum, mul
 
 def test_sum():
     a = 1
@@ -15,3 +15,10 @@ def test_sum_wrong():
     s_wrong = 4
 
     assert s_wrong == sum(a, b)
+
+def test_mul():
+    a = 1
+    b = 2
+    m = 2
+
+    assert m == mul(a, b)