More unit tests.

test.py

@@ -2,8 +2,18 @@
 
 # import pytest
 
-from calculator import add
+from calculator import add, subtract, multiply, divide
+
 
 def test_add():
     assert add(2, 2) == 4
 
+def test_subtract():
+    assert subtract(4, 2) == 2
+
+def test_multiply():
+    assert multiply(4, 4) == 16
+
+def test_divide():
+    assert divide(8, 2) == 4
+