I think there's another problem: it fails to install gcc-mingw-w64-i686 because autopkgtest tries to install gcc-mingw-w64-i686:i386 which does not exist since it only exists as gcc-mingw-w64-i686:all.
I added :all to the test dependency, and then it works, the test passes correctly on i386 \o/
But I'll come up with the proper patch in autopkgtest: it should try :$arch first and then :all in case it does not work.
I think there's another problem: it fails to install gcc-mingw-w64-i686 because autopkgtest tries to install gcc-mingw- w64-i686: i386 which does not exist since it only exists as gcc-mingw- w64-i686: all.
I added :all to the test dependency, and then it works, the test passes correctly on i386 \o/
But I'll come up with the proper patch in autopkgtest: it should try :$arch first and then :all in case it does not work.