Before this commit, when navigating from test environment (e.g.
page reload), it crashed with following error:

`TypeError: Cannot convert undefined or null to object`

This error comes from mocked bus services in tests: even when they
have been destroyed, they handle the window event 'unload'. They
no longer have a parented parent, so `this.call()` returns
`undefined`, hence the crash.

This commit prevents mocked bus services to listen on window 'unload'
event. Tests must always simulate this window event by explicitly
calling the handler.

closes odoo/odoo#41374

X-original-commit: fd08d04f
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>