PURPOSE
Have a cleaner test_mail addons
SPECIFICATIONS
Keep only mail-related tests, move odoobot in test mail full, send "update
notification" tests in mail (specific to mail). Merge some test files to
lessen number of files, perform light file renaming.
Split test mail models file to prepare some cleaning in those models and tests.
LINKS
Prepares Task ID 2238597 (clean notification models)
Prepares Task ID 2083854 (improve mass mailing technical flows)
PR #49891