[FIX] base: a traceback appear when we test SMTP connection in technical settings

Before this commit, in base we use bus to notify the user but bus module
is not always installed and so we can't call it.

After this commit, now we use a client action to notify the success test.

Steps to reproduce:
* install a Odoo with no apps
* go in debug mode
* go to "Outgoing Mail Servers" technical settings
* create a valid SMTP record
* click on "Test Connection" button (BUG)

closes odoo/odoo#38276

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>