Ensure duplicate menu instances are merged

This is sort of a recurrance of an issue I introduced in
#33037 and then fixed in
#33271 See that second PR
description for details

This was then broken in this commit:
8db8f4a
The test created in that second PR above demonstrated that instances in
the second of two modules with the same ID will be propagated to all
forms in both modules, but it neglected to verify that same property of
instances required by the first of those two modules.  That latest commit above
had subsequent menus overwriting instances from earlier ones.

corehq/apps/app_manager/suite_xml/post_process/instances.py

@@ -172,7 +172,7 @@ def _menu_xpaths_by_command(self):
         # multiple menus can have the same ID - merge them first
         xpaths_by_menu_id = defaultdict(set)
         for menu in self.suite.menus:
-            xpaths_by_menu_id[menu.id] = menu.get_all_xpaths()
+            xpaths_by_menu_id[menu.id].update(menu.get_all_xpaths())
 
         return defaultdict(set, {
             command.id: xpaths_by_menu_id[menu.id]

corehq/apps/app_manager/tests/test_suite_instances.py

@@ -271,14 +271,18 @@ def test_module_filter_instances_on_all_forms_merged_modules(self):
         # and if two modules are display_in_root, then on forms in the other module too
         factory = AppFactory(build_version='2.20.0')  # enable_module_filtering
         self.m0, self.m0f0 = factory.new_basic_module('m0', 'case1')
+        self.m0.module_filter = "instance('groups')/groups/"
         self.m0.put_in_root = True
 
         self.m1, self.m1f0 = factory.new_basic_module('m1', 'case1')
         self.m1.module_filter = "instance('locations')/locations/"
         self.m1.put_in_root = True
 
         suite = factory.app.create_suite()
-        instance_xml = "<partial><instance id='locations' src='jr://fixture/locations' /></partial>"
+        instance_xml = """<partial>
+            <instance id='groups' src='jr://fixture/user-groups' />
+            <instance id='locations' src='jr://fixture/locations' />
+        </partial>"""
         self.assertXmlPartialEqual(instance_xml, suite, "entry/command[@id='m0-f0']/../instance")
         self.assertXmlPartialEqual(instance_xml, suite, "entry/command[@id='m1-f0']/../instance")