Consider using non-Riseup email for Thunderbird automated tests

The test suite config we have in “shared-secrets” and the one used by our Jenkins workers all use Riseup email accounts for testing Thunderbird. Problem is, the config for Riseup is shipped with Thunderbird itself, so we effectively don’t test the network-based part of the Thunderbird account setup wizard, which worries me since 1. that’s the part we patch; 2. presumably we don’t test the most common use case.

Should we switch to non-Riseup email accounts for these tests?

Related issues

• Related to #6156

Original created by @intrigeri on 16856 (Redmine)