Add a job to run 'uscan' and test if debian/watch works correctly

While browsing debaudit I've noticed on almost 20% of packages in Debian running uscan --download-current-version fails.

Some people are not using uscan at all to fetch new upstream versions and the debian/watch is totally unmainteined. In some cases it is just a little bit broken and recently there have been a lot of people converiting their debian/watch to version 5 to silence the Lintian bug, but they did it after imporing a new upstream version and actually never test and validate that it works yet upload it to Debian among other packaging changes.

Having a job that simply runs uscan --download-current-version --version will validate if uscan works, and emit log showing what went wrong in case it didn't work.