d/rules: build tests in parallel as well
The program is built with dh_auto_build, which parallelises per dpkg-buildpackage -j automatically.
However, the dh_auto_test override simply runs $(MAKE), which forwards the job control from d/rules, which is unset. (It's only set with --jobs-force.)
This meant that building the tests took five minutes (longer than the rest) instead of 20 seconds. Forward parallel= from DEB_BUILD_OPTIONS set by dpkg-buildpackage.
Also the clean target left artifacts in .build.