Enable unit tests in GitHub Actions and add initial test cases for add_numbers function

Author: austinsonger

.github/workflows/python.yaml

@@ -23,6 +23,6 @@ jobs:
           run: ruff check .
         - name: Check formatting with black
           run: black . --check --verbose
-#        - name: Run unit tests
-#          run: |
-#            pytest
+        - name: Run unit tests
+          run: |
+            pytest

tests/main_test.py

@@ -0,0 +1,8 @@
+from main import add_numbers
+
+
+def test_add_numbers():
+    assert add_numbers(1, 2) == 3
+    assert add_numbers(-1, 1) == 0
+    assert add_numbers(0, 0) == 0
+    assert add_numbers(-1, -1) == -2