Won't this affect the regular code, though?

Yes, I think so and I'm unsure what is the outcome of such a change. Can I check if we are in TestMode and disable this only when on testMode?

qputenv in the test and qgetenv here.
Or make the test set an extern bool to true. See e.g. kio_resolve_local_urls in kio.

Or make the test closer to the regular case ;-)
In fact I'm wondering why this says setReadDefaults(true), i.e. ignore user settings. That's usually for "Revert to defaults" buttons.

qputenv in the test and qgetenv here.
Or make the test set an extern bool to true. See e.g. kio_resolve_local_urls in kio.

thx for the tips.

Or make the test closer to the regular case ;-)

Well I want the tests to be working without installation and as non root I can't write to /usr/share. So I would need to control the default location and I don't see a way to achieve this.

In fact I'm wondering why this says setReadDefaults(true), i.e. ignore user settings. That's usually for "Revert to defaults" buttons.

This I'm wondering, too. I tracked this line of code exists from the beginning of the git history and this file was added 2004 and that line was never touched since then. The commit message is:
"Add optional graphical spam status to fancy headers. Based on patch by Patrick Audley." commited by @ikloecker. But I expect Ingo also don't know why this line is needed.

I would suggest to remove this line as it seems to me, that it breaks stuff (e.g. our tests) and I can't imagine what we gain if we keep this line intact. With removing the line we gain the possibility for users to change the "kmail.antispamrc" for their needs.