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-attribute-value-unicode.yml

@@ -1,16 +1,29 @@
 ---
   - 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/rating/@subjective' to 'нет'
     xml:
       path: /tmp/ansible-xml-beers.xml
       xpath: /business/rating
       attribute: subjective
-      value: "нет"
+      value: нет
+    register: set_attribute_value_unicode
+
+  - name: Compare to expected result
+    copy:
+      src: results/test-set-attribute-value-unicode.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-attribute-value-unicode.xml /tmp/ansible-xml-beers.xml
-    changed_when: False
+    assert:
+      that:
+      - set_attribute_value_unicode.changed == true
+      - comparison.changed == false  # identical
+    #command: diff -u {{ role_path }}/results/test-set-attribute-value-unicode.xml /tmp/ansible-xml-beers.xml