[FIX] web: make broken test in firefox pass

The attribute `autocomplete` is set with the jQuery function `attr` but
the test was using the function `prop`.

This seems to work in Chrome but apparently not in Firefox.

addons/web/static/tests/views/form_tests.js

@@ -5593,9 +5593,9 @@ QUnit.module('Views', {
         form.$buttons.find('.o_form_button_edit').click();
         assert.strictEqual(form.$('input[name="foo"]').val(), '***',
             "password should be displayed with stars");
-        assert.strictEqual(form.$('input[name="display_name"]').prop('autocomplete'), 'coucou',
+        assert.strictEqual(form.$('input[name="display_name"]').attr('autocomplete'), 'coucou',
             "attribute autocomplete should be set");
-        assert.strictEqual(form.$('input[name="foo"]').prop('autocomplete'), 'new-password',
+        assert.strictEqual(form.$('input[name="foo"]').attr('autocomplete'), 'new-password',
             "attribute autocomplete should be set to 'new-password' on password input");
         form.destroy();
     });