[FIX] product: do not create duplicate variants

Create a product template Pt with attibutes A1, A2,
respectively with values Va1_1, Va1_2, and Va2.
This should make 2 variants, V1 and V2.
Sell some of these variants.
Then they can't be deleted since they are linked to orders/invoices/...

Now delete attribute A2. We know that:
Warning: adding or deleting attributes will delete and recreate existing
variants and lead to the loss of their possible customizations.

In fact, old variants are only archived instead of deleted:
for variant in variants_to_unlink: ... variant.write({'active': False})
as a fallback.

Now add again the attribute A2 to Pt.
It creates a new variant with the correct attributes.
Since the product is not active and has valid attributes and attribute values,
it is reactivated.
As a result we get the same variant twice.

Here we can simply add a check: if there is already an existing variant with
the same values, we can skip this activation.

We add a tiny little test to make sure that we have correct number of variants.
It's basically unreadable but somewhat commented.

fix coauthored with @rco-odoo

opw 2029985

closes odoo/odoo#35013

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>