[FIX] web: MockServer: search_read: handle sort on many2one

Before this commit, if the records had to be sorted according a
many2one field, and there was a sequence field on the many2one
comodel, the sequence was ignored. Now, the sequence is taken into
account like the actual server does.

Linked to odoo/enterprise/pull/7407

closes odoo/odoo#42595

Related: odoo/enterprise#7407
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>