ci: run the unittest suite on push and pull requests

The repository already ships 70 stdlib unittest cases under tests/, but
.github/workflows/build.yml only builds PyInstaller binaries and images,
so a regression never fails a PR. This adds a tiny Tests workflow that
installs requirements.txt and runs `python -m unittest discover -s tests`
on pushes to main, on every pull request and on demand.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -0,0 +1,30 @@
+name: Tests
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  unittest:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install -r requirements.txt
+
+    - name: Run unit tests
+      run: python -m unittest discover -s tests -v