Map "Fixed positioning" to web-features (#56500)

Author: howard-e

css/CSS2/positioning/WEB_FEATURES.yml

@@ -0,0 +1,6 @@
+features:
+- name: fixed-positioning
+  files:
+  - 'position-fixed-*.xht'
+  - position-004.xht
+  - position-006.xht

css/css-position/WEB_FEATURES.yml

@@ -8,6 +8,9 @@ features:
   - backdrop-inherit-rendered.html
   - backdrop-tree-abiding-slotted.html
   - replaced-object-backdrop.html
+- name: fixed-positioning
+  files:
+  - 'position-fixed-*'
 - name: z-index
   files:
-  - '*z-index*'
+  - '*z-index*'