Test UserNotes\Sorter

We add a copy of the latest run-tests.php from the PHP-8.0 branch, and
tests for the `UserNotes\Sorter` with full code coverage.  We also run tests
on GitHub Actions.

Co-authored-by: Andreas Möller <am@localheinz.com>

Closes GH-606.

.github/workflows/integrate.yaml

@@ -0,0 +1,37 @@
+# https://docs.github.com/en/actions
+
+name: "Integrate"
+
+on:
+  pull_request: null
+  push:
+    branches:
+      - "master"
+
+jobs:
+  tests:
+    name: "Tests"
+
+    runs-on: "ubuntu-18.04"
+
+    strategy:
+      matrix:
+        php-version:
+          - "7.3"
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v3"
+
+      - name: "Set up PHP"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          coverage: "none"
+          extensions: "none"
+          php-version: "${{ matrix.php-version }}"
+
+      - name: "Set up problem matchers for PHP"
+        run: "echo \"::add-matcher::${{ runner.tool_cache }}/php.json\""
+
+      - name: "Run tests"
+        run: "php tests/run-tests.php -j3 -q --show-diff"