tests: Avoid dch's unknown distribution warning

As part of the test pipeline we add a debian/changelog entry with the distribution set to apertis. Since we do that from a Debian container, debchange complains with:

dch warning: Unable to determine Debian's backport distributions.
dch warning: Recognised distributions are:
experimental, unstable, testing, stable, oldstable, oldoldstable,
{bullseye,buster,stretch,jessie,wheezy}-proposed-updates,
{testing,stable,oldstable,oldoldstable}-proposed-updates,
{bullseye,buster,stretch,jessie,wheezy}-security,
{testing,stable,oldstable,oldoldstable}}-security and UNRELEASED.
Using your request anyway.
dch: Did you see that warning?  Press RETURN to continue...

Telling dch that we're targeting a different vendor silences the warning.