[FIX] web: mock fieldsViewGet don't delete raw attrs

Have a view with a field as:
<field name="stuck_in_the_middle" attrs="{'readonly': [('with_you', '=', False)]}"/>

Before this commit, the contents of attrs were parsed and merged into the modifiers
and the attrs attribute of the node was deleted
which might cause divergence between reality and tests

After this commit, the mock server does what the server does in this case

closes #25583

addons/web/static/tests/helpers/mock_server.js

@@ -229,7 +229,6 @@ var MockServer = Class.extend({
             if (node.attrs.attrs) {
                 var attrs = pyeval.py_eval(node.attrs.attrs);
                 _.extend(modifiers, attrs);
-                delete node.attrs.attrs;
             }
             if (node.attrs.states) {
                 if (!modifiers.invisible) {