tests: activate dpkg multi-arch even if test is single arch

tests: activate dpkg multi-arch even if test is single arch

Most tests are either multiarch, do not care for the specific
architecture or do not interact with dpkg, so really effect by this is
only test-external-installation-planner-protocol, but its a general
issue that while APT can be told to treat any architecture as native
dpkg has the native architecture hardcoded so if we run tests we must
make sure that dpkg knows about the architecture we will treat as
"native" in apt as otherwise dpkg will refuse to install packages from
such an architecture.

This reverts f883d2c3675eae2700e4cd1532c1a236cae69a4e as it complicates
the test slightly for no practical gain after the generic fix.