PURPOSE
Cleanup and improve composer-related tests. As composer will soon be improved
better ensure we don't break flows.
SPECIFCIATIONS
Cleanup existing tests: rename, make it use tools, reoder them.
Add some tests about composer in order to better spot changes and differences
as well as to help debugging mail composer code update. Notably add tests
with Form tool, add tests for reply-to computation, auto-delete, mail server.
Performance tests about composer are added, notably using the Form tool. This
helps determining differences between creating / manipulating a composer in
code and in UI. Indeed notably onchange and some side effects may lead to
different query counters, which is the case as highlighted here. Tests
involving attachments are also added to see their impact on posting process.
Task-2673913 (TestMail: Cleanup and improve test coverage)
Part-of: odoo/odoo#86393