Fix crash caused by bl_keymap_validate clearing key-maps

The fix for disabling add-ons #113206 caused a crash when the
bl_keymap_validate test exited.

This happened because add-on unregister would try to remove key-map
items that had been freed.

Resolve by disabling all addons instead of clearing the keymaps.

tests/python/bl_keymap_validate.py

@@ -273,10 +273,8 @@ def main() -> None:
     relaxed = "--relaxed" in argv
 
     # NOTE(@ideasman42): Disable add-on items as they may cause differences in the key-map.
-    # An alternative would be to disable all add-ons, but this is simpler.
-    if kc_addon := bpy.context.window_manager.keyconfigs.addon:
-        kc_addon.keymaps.clear()
-    del kc_addon
+    import addon_utils
+    addon_utils.disable_all()
 
     has_error = False