Add some selfupdate integration tests

We just start with the 4.1.8 release that was
the first supporting the selfupdate command.

Then we upgrade to actual and then rollback
to confirm that we are back to the original one.

That should be enough to have it continuously covered.

.github/workflows/test.yml

@@ -261,3 +261,45 @@ jobs:
         php build/moodle-plugin-ci.phar behat --profile default
         php build/moodle-plugin-ci.phar behat --profile chrome
         php build/moodle-plugin-ci.phar behat --profile firefox --tags="@local_ci&&~@app"
+
+  selfupdatetest:
+    name: SelfUpdate tests (PHAR)
+    runs-on: ubuntu-22.04
+    env:
+      lowest_release: '4.1.8'
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Each supported PHP version once. That's enough.
+          - php: '8.2'
+          - php: '8.1'
+          - php: '8.0'
+          - php: '7.4'
+
+    steps:
+    - name: Setup PHP ${{ matrix.php }}
+      uses: shivammathur/setup-php@v2
+      with:
+        php-version: ${{ matrix.php }}
+        extensions: pgsql, zip, gd, xmlrpc, soap
+        ini-values: max_input_vars=5000
+        coverage: none
+
+    - name: Download base ${{ env.lowest_release }} PHAR artifact
+      uses: robinraju/release-downloader@v1.8
+      with:
+        repository: moodlehq/moodle-plugin-ci
+        tag: ${{ env.lowest_release }}
+        fileName: moodle-plugin-ci.phar
+
+    - name: Self update PHAR to actual
+      run: |
+        php moodle-plugin-ci.phar selfupdate
+        php moodle-plugin-ci.phar --version | grep -qv "${{ env.lowest_release }}"
+
+    - name: Self update rollback PHAR to base ${{ env.lowest_release }}
+      run: |
+        php moodle-plugin-ci.phar selfupdate --rollback
+        php moodle-plugin-ci.phar --version | grep -q "${{ env.lowest_release }}"