tests: make custom templates that use {whyunstable} terser
These templates demonstrate that {whyunstable} is fully template-friendly, but
they don't need to produce such long lines of output.
FROM ubuntu:__CODENAME__
RUN apt-get update && apt-get install -y \
build-essential \
debhelper \
devscripts \
dh-python \
less \
python \
python-all-dev \
python-docutils \
unzip \
zip