Create mock webserver and proper mock sds011

[pgdr]

We want to be able to test

1. FriskbySampler by giving it a mock object to sample from, e.g. MockSds011 which is currently located in run_tests.
2. FriskbySubmitter should be given either a URL to a local friskby database, or in other ways a mock server.

[semafor]

Given the FriskbySampler case, where the sampler wants to ask sensors for data, I suggest that FriskbySampler uses inter-process communication (IPC) to query the sensors. A couple of reasons for this:

• In my limited experience, it's the only way to prevent some serious bit rot, where mock code rots proportionally fast to the amount of other “real” tasks, because it has to change almost as often as the real code.
• There's an API which is extremely well defined (in XML).
• In the long run, other apps/services we (or the community/others) create, do not have to do a python import, but can rather use general purpose IPC tooling.
• It is almost trivial to create mocking for IPC (e.g. for python, take a look at https://github.com/martinpitt/python-dbusmock/).

This is, of course, a large investment to solve a seemingly “easy” testing problem, but it will definitely pay off in the long run (take look at dbus-monitor [--system | --session] on a linux system utilizing dbus, and note the services that operate within that space, e.g. NetworkManager, systemd, all org.freedesktop.* services, etc.).