[FIX] web: Make sample server tests deterministic

Before this commit, there were a few tests in the "sample server" suite
that asserted that every group resulting of a "read_group" would contain
at least one record, while there was actually a slight chance of it not
happening.

Now, the sample server used in the tests has been rendered deterministic
and these kind of issues are completely avoided.

closes odoo/odoo#54164

X-original-commit: a6d32ff46558b68ce3f19458f29c311ee5c658fd
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
Signed-off-by: Julien Mougenot (jum) <jum@odoo.com>