add unit test for additive and substractive

test_parse_plus_or_minus.py

@@ -0,0 +1,31 @@
+from calc_str_eval import parse_add_or_minus
+
+
+def main():
+    try:
+        correct: int = 1
+        equation: str = "0+1"
+        result = parse_add_or_minus(equation)
+        assert correct == result, f"want {correct}, got {result}"
+
+        correct: int = -1
+        equation: str = "0-1"
+        result = parse_add_or_minus(equation)
+        assert correct == result, f"want {correct}, got {result}"
+
+        correct: int = 1
+        equation: str = "2-1"
+        result = parse_add_or_minus(equation)
+        assert correct == result, f"want {correct}, got {result}"
+
+        correct: int = 5
+        equation: str = "2+3"
+        result = parse_add_or_minus(equation)
+        assert correct == result, f"want {correct}, got {result}"
+
+    except AssertionError as err:
+        print("AssertionError :", err)
+
+
+if __name__ == "__main__":
+    main()