Running functional tests against a remote server

Currently, this functional test suite needs full command and file system access to the tested Heptapod server.

There are several reasons for this, some being structural, like loading corruption cases that we can't reproduce any more from tarballs, some that will soon be obsolete, like tweaking HGRCs (see heptapod#165 (closed))

Still, quite a few tests could be run without any such kind of access, and it is infinitely better to skip the other ones than not to be able to run any test. This will be for instance important to validate new deployments, such as the future foss.heptapod.net