[IMP] digest: add tests about tone down and unsubscribe

Purpose is to ensure behavior of digests and prepare future fixes and
improvements. Tone down and unsubscribe links are currently not tested.
This commit fixes that by adding relevant tests.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onbarding and usage improvement)

Part-of: odoo/odoo#79653