CI: Add XMPP Interop tests

Modifies the CI build to integrate the GitHub Action-based test runner from the [XMPP Interop Testing project](https://xmpp-interop-testing.github.io). This executes additional integration tests that help verify ejabberd's compliance with XMPP specifications.

In this commit, none of the tests are excluded, which likely results in false positives. Therefor, the 'continue-on-error' flag is set. This should ideally be removed in a later commit.

.github/workflows/ci.yml

@@ -147,12 +147,28 @@ jobs:
         cat $RE/logs/ejabberd.log
         grep -q "is stopped in" $RE/logs/ejabberd.log
 
-    - name: Check Development Release
+    - name: Start Development Release
       run: |
         make dev
         RE=_build/dev/rel/ejabberd
+        sed -i 's/starttls_required: true/starttls_required: false/g'  $RE/conf/ejabberd.yml
         $RE/bin/ejabberdctl start
         $RE/bin/ejabberdctl started
+        $RE/bin/ejabberdctl register admin localhost admin
+        grep -q "is started in" $RE/logs/ejabberd.log
+
+    - name: Run XMPP Interoperability Tests against CI server.
+      continue-on-error: true
+      uses: XMPP-Interop-Testing/xmpp-interop-tests-action@v1.4.0
+      with:
+        domain: 'localhost'
+        adminAccountUsername: 'admin'
+        adminAccountPassword: 'admin'
+
+    - name: Stop Development Release
+      if: always()
+      run: |
+        RE=_build/dev/rel/ejabberd
         $RE/bin/ejabberdctl stop
         $RE/bin/ejabberdctl stopped
         cat $RE/logs/ejabberd.log