TestMail holds a tool to create test records in batch, notably with inline partner creation. Allow its usage in some sub modules and tests depending directly from the base mail test class. This allows notably usage of this tool in performance tests (see enterprise PR). Also explicitly set some test data to ease sub tests writing. Prepares Task-2150462 (Mass Mailing: Unsubscribe flow improvement) Part-of: odoo/odoo#94660