[FIX] mail, web: directly load async debug.xml during QUnit tests

Before this commit, the test `list: edit view menu item` would sometimes crash
as now the debug manager loads its templates with an `xmlDependencies` where it
was not the case before (included in base.xml), see 73327db065.

addons/mail/static/tests/tools/debug_manager_tests.js

@@ -28,7 +28,7 @@ QUnit.module('Mail DebugManager', {}, function () {
             },
         });
 
-        debugManager.appendTo($('#qunit-fixture'));
+        await debugManager.appendTo($('#qunit-fixture'));
 
         // Simulate update debug manager from web client
         var action = {

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

@@ -102,6 +102,7 @@ async function nextTick() {
 // to load xml files that are normally lazy loaded by specific widgets).
 return Promise.all([
     session.is_bound,
+    ajax.loadXML('/web/static/src/xml/debug.xml', core.qweb),
     ajax.loadXML('/web/static/src/xml/dialog.xml', core.qweb)
 ]).then(function () {
     setTimeout(function () {

addons/web/static/tests/tools/debug_manager_tests.js

@@ -13,7 +13,7 @@ QUnit.module('DebugManager', {}, function () {
 
         var debugManager = createDebugManager();
 
-        debugManager.appendTo($('#qunit-fixture'));
+        await debugManager.appendTo($('#qunit-fixture'));
 
         // Simulate update debug manager from web client
         var action = {