[FIX][IMP] test_mail_full: improve tests for rendering in mass mode

Purpose is to enable dynamic rendering in mass comment mode. This highlights
an issue, as currently when using the comment mode on several records body
is not rendered, just sent as it is. This means raw dynamic code could be
sent.

Also cleanup some invalid setup (not enough records for mass) that skipped
some tests.

Spotted during Task-2613245 (Server actions mail update / cleaning)

Part-of: odoo/odoo#97734