[FIX] mrp: check cycle with 'apply on' feature

Commit [1] does not include the amazing feature "apply on variants".
As a result, a legit configuration could lead to a `ValidationError`
(see the new test `test_cycle_on_legit_apply_variants`)

Note about the modifications of `test_11_multi_level_variants`:
because of an ORM limitation, if we create a BoM line directly:
```py
self.env['mrp.bom.line'].create({...})
```
the constraint will not be triggered... We need to write the line
creation directly on the BoM

[1] 4a4f92c5

opw-3200969

closes odoo/odoo#133039

Signed-off-by: William Henrotin (whe) <whe@odoo.com>