Add manually triggered unit test workflow

.github/workflows/tests.yml

@@ -0,0 +1,34 @@
+name: Unit tests
+
+# Manually triggered only - so tests can be run once PR comments are resolved,
+# instead of on every push.
+#
+# Two ways to trigger:
+#   1. Add the "run-tests" label to a pull request (re-add it to re-run).
+#   2. Actions -> "Unit tests" -> "Run workflow" and pick a branch.
+on:
+  pull_request:
+    types: [labeled]
+  workflow_dispatch:
+
+jobs:
+  test:
+    # For the label trigger, only react to the "run-tests" label.
+    if: github.event_name == 'workflow_dispatch' || github.event.label.name == 'run-tests'
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: "10.0.x"
+
+      - name: Restore
+        run: dotnet restore Shoebox.slnx
+
+      - name: Test
+        run: dotnet test Shoebox.slnx --configuration Release --no-restore