CI: run the test suite on every PR and push to main

Ubuntu runner, Python 3.11 and 3.13 (stdlib-only project: oldest
supported and newest current). Git identity and init.defaultBranch=main
configured for the tests that build scratch repos; everything else the
suite needs (bash, tar, rsync, stub binaries) is on the runner already.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/tests.yml

@@ -0,0 +1,28 @@
+name: tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        # Stdlib-only, no install: the suite must pass on the oldest
+        # Python bench supports and the newest current one.
+        python-version: ["3.11", "3.13"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Configure git for the tests that build scratch repos
+        run: |
+          git config --global user.name ci
+          git config --global user.email ci@bench.invalid
+          git config --global init.defaultBranch main
+      - run: python3 -m unittest discover -s tests -v