Skip tests that require Internet access by default

Ubuntu has a quality control policy that states that package tests
should be run automatically as part of a package build. However, in
order to ensure that builds and tests are reproducible from package
source code only, Internet access is disabled in our build
infrastructure.

Therefore, do not run tests that require Internet access by default.

This might need some work, since I presume you do want the option of running these tests somehow. Presenting this for feedback. I'll cherry-pick this patch into Ubuntu now, in order to get things going.