[pkg] Add new autopkgtest for skeleton packaging
When running `cubicweb-ctl newcube`, the generated template provides
debian packaging. So let's add automated tests for that using
autopkgtest.
The new test will generated a new cube, build a source tarball,
build the package, try to install it, see if the Python 2 and 3
modules are available and even run autopkgtest against the newly
created packages.
Along the way it will also print information about the generated
package: buildinfo, package content, and lintian report.