[FIX] mail:tests: restore unintentionally remove test

Revision on https://github.com/odoo/odoo/commit/ff7ea6b654175e3138f5f36d2e5b4214cebef499



Due to some git conflicts, it duplicated history tests and removed
the previous test 'input not cleared on unresolved message_post rpc'.

closes odoo/odoo#35502

Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>