[IMP] test_mail(_full): run tests at post_install

- take advantage of runbot multi-build capabilities
- get similar result when running them locally during dev
- better detect when other modules add extra queries

Query counts are split in the base value (testing with just test_mail installed)
+ the extra modules overhead.

Part of task-2178641

closes odoo/odoo#43666

Pr: #43666
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>