Extensions: resolve test failing

Enable the extensions add-on by default, without this, it's enabled
on startup, causing the preferences to be tagged as modified.

scripts/addons_core/bl_pkg/tests/test_cli_blender.py

@@ -403,7 +403,7 @@ class TestWithTempBlenderUser_MixIn(unittest.TestCase):
             "--clear-all",
             repo_id,
         ))
-        self.assertEqual(stdout, "Info: Preferences saved\n")
+        self.assertEqual(stdout, "")
 
     def build_package(
             self,

source/blender/blenkernel/intern/blendfile.cc

@@ -1445,6 +1445,7 @@ UserDef *BKE_blendfile_userdef_from_defaults()
         "io_scene_gltf2",
         "cycles",
         "pose_library",
+        "bl_pkg",
     };
     for (int i = 0; i < ARRAY_SIZE(addons); i++) {
       bAddon *addon = BKE_addon_new();