xml module: Better change detection, improved tests (#28460)

This PR includes:

- Improvements to change-detection by comparing 2 objectified XML trees
- Implement better integration tests by comparing 2 files using copy

test/integration/targets/xml/tasks/test-set-element-value-empty.yml

@@ -1,14 +1,28 @@
 ---
   - name: Setup test fixture
     copy:
-      src: '{{ role_path }}/fixtures/ansible-xml-beers.xml'
+      src: fixtures/ansible-xml-beers.xml
       dest: /tmp/ansible-xml-beers.xml
 
-  - name: Set /business/website/address to empty string.
+
+  - name: Set '/business/website/address' to empty string.
     xml:
       path: /tmp/ansible-xml-beers.xml
       xpath: /business/website/address
       value: ''
+    register: set_element_value_empty
+
+  - name: Compare to expected result
+    copy:
+      src: results/test-set-element-value-empty.xml
+      dest: /tmp/ansible-xml-beers.xml
+    check_mode: yes
+    diff: yes
+    register: comparison
 
   - name: Test expected result
-    command: diff -u {{ role_path }}/results/test-set-element-value-empty.xml /tmp/ansible-xml-beers.xml
+    assert:
+      that:
+      - set_element_value_empty.changed == true
+      - comparison.changed == false  # identical
+    #command: diff -u {{ role_path }}/results/test-set-element-value-empty.xml /tmp/ansible-xml-beers.xml