[IMP] crm: clean and improve tests for lead2opportunity converters

PURPOSE

As crm will soon evolve (onchange -> compute, code improvements, addition of
new features) cleaning and improving tests is necessary to help avoid issues.

SPECIFICATIONS

In this commit we add tests for the lead 2 opportunity converters. Indeed
it is strange that such a critical wizard has so few tests. Notably the
use of single-lead / multi-lead mode is more tested, allowing to see that
code is somewhat oldish.

LINKS

Side effect of Task ID 2056759 (remove crm.partner.binding mixin)
Side effect of Task ID 2088565 (crm onchange -> compute)
Community PR #43127
Enterprise PR odoo/enterprise#7656