Add unit tests

.github/workflows/tests.yaml

@@ -0,0 +1,18 @@
+name: tests
+on: [push, pull_request]
+jobs:
+  unit-tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Setup
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.10'
+      - name: Test
+        run: |
+          export PYTHONPATH=.
+          python3 -m venv venv
+          venv/bin/pip install --upgrade pip
+          venv/bin/pip install pytest mockito
+          venv/bin/pytest test/test_on-modify_unit.py