install_mirror_package: When arch is set, install both native and arch

This is necessary as dpkg multiarch does not allow mismatching versions
on different architectures (e.g. you cannot have libsbc1-1.0:i386 and
libsbc1-1.1:amd64 installed at the same time).

Even --force-all in dpkg does not allow that.

We fix the problem by installing both packages at the same time in
install_mirror_package.
test/tests/30-audio: Test that install_mirror_package is able to update libsbc1.

Force installing a newer version, and see if it gets downgraded.