testsuite require network access by default

Created by: jonassmedegaard

The test file tests/test_doctests.py fails in a build environment without network access.

Some build environments - including Debian autobuilders - explicitly disable network to make builds more deterministic.

Please rely on network access only optionally. One elegant way to do that is to borrow the logic used in Perl: Check environment variable EXTENDED_TESTING and if not set to a non-empty value then skip all such "extended testing".

Thanks for considering.