[FIX] base: correctly check debug mode when formatting access errors

Intent of checking `group_no_one` was always to query the advanced
info / debug mode, however when the semantics of group_no_one got
changed in 31518bc0 this site was
missed, and now always displays "advanced" errors for internal
users. Which was not the intent.

Also since we're printing `display_name` and some of them annoyingly
hook onto context variables to show extended information, reset the
context to the user's default in order to avoid such
extended-formatting `display_name`.

Also fix "debug mode" in `TestIRRuleFeedback`, which has been broken
since time immemorial (likely as long as the group_no_one semantics
changed).

closes odoo/odoo#135028

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>