tests/: add tos-gate test and CI workflow

29 tests across four classes:
- TestTosGatedDownloads (9) - macro unit tests: URL absent from HTML,
  base64/JSON roundtrip, group format encoding, checkbox attributes,
  prose_section_id, tos_url override
- TestTosCheckbox (4) - legacy macro: element presence, section ID, URL
  defaults/override
- TestTosGateJs (7) - JS file: function defined, noreferrer absent from link
  template but warning comment present, data-prose-group selector present, group
  branch present, prose hidden on uncheck
- TestStaticFiles (9) - no plain dl.3mdeb.com links, correct macro usage in
  variants/ files, tos-gate.js in mkdocs.yml, permalink: true, referrer meta in
  overrides/main.html

Signed-off-by: Artur Raglis <artur.raglis@3mdeb.com>

.github/workflows/test-tos-gate.yml

@@ -0,0 +1,38 @@
+---
+name: ToS gate tests
+
+on:
+  push:
+    branches:
+      - master
+      - main
+    paths:
+      - main.py
+      - docs/javascripts/tos-gate.js
+      - overrides/main.html
+      - mkdocs.yml
+      - docs/**
+      - tests/test_tos_gate.py
+      - requirements.txt
+  pull_request:
+    paths:
+      - main.py
+      - docs/javascripts/tos-gate.js
+      - overrides/main.html
+      - mkdocs.yml
+      - docs/**
+      - tests/test_tos_gate.py
+      - requirements.txt
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.x"
+      - name: Install dependencies
+        run: pip install -r requirements.txt
+      - name: Run ToS gate tests
+        run: pytest tests/test_tos_gate.py -v