Track untestable features and exclude them from feature coverage reports

[queengooborg]

This fixes #1137.

[Elchi3]

I wonder if we should limit this list to untestable features in the sense that we only list untestable things that require specific hardware etc. For things like worker support, it seems that we can test these one day, but work on the collector is required, right? I think those shouldn't be classified as "untestable".

[queengooborg]

The list is separated into two sections, with comments annotating each one: the truly untestable features, and the features that are testable but would require an overhaul of collector architecture. Worker support is one of those overhaul-requiring features.

While it's a misnomer to put it in the "untestable features" list, I don't think that it's worth trying to implement support for those kinds of features right now, so that's why I put them in the list!