So how can I sneak in some Test-Depends that are needed for the test?
I think this is what you seek:
Simply add a d/tests/autopkgtest-pkg-r.conf
This is documented also in the autodep8(1) manpage.
I recently found out about this too
Or may be it makes sense to always feed r-cran-testthat into the test
environment?
I don't think that'd be a very good solution. IMO adding a conf file is straightforward anyway.
But I've no strong opinions here.
Nilesh