Add unit tests for calculator

CalculatorLibrary/calculator.py

@@ -0,0 +1,11 @@
+"""
+Calculator library containing basic math operations.
+"""
+
+
+def add(first_term, second_term):
+    return first_term + second_term
+
+
+def subtract(first_term, second_term):
+    return first_term - second_term