[18.0][FIX] account_invoice_auto_send_by_email: set trusted outgoing bank in tests

[grojof]

Summary

• Set the test bank account as allowed for outgoing payments in account_invoice_auto_send_by_email tests.
• Prevent setup failures when validating invoice email sending behavior.

Why

The test setup must use a bank account that is valid for outgoing payments in this scenario.

Changes

• account_invoice_auto_send_by_email/tests/test_account_invoice_auto_send_by_email.py

Merge order

1/4

Related PRs (recommended order):

• [18.0][FIX] tests: stabilize account_billing/custom_rounding/show_currency_rate #2272
• [18.0][FIX] account_invoice_custom_rounding: keep tax base-line contract #2273
• [18.0][FIX] sale_stock_picking_invoicing: stabilize matrix tests #2274

[grojof]

Cross-reference: migration follow-up has been posted on #2063 for rerun with the updated 18.0 context (after #2271-#2274 stabilization).

Ref: #2063 (comment)