ws: Return hard coded Content-Type based on extension

Looking up Content-Type via GFileInfo isn't working. In addition
if it was to work it would invoke a whole lot of machinery.

We only serve a limited set of files. Hard code content type based
on extension. This is efficient and predictable.

Fixes #225
ws: Make note of skipped tests due to missing system bus

This allows the TAP output to reflect the skipped test.
ws: Return correct Content-Type from cockpitdyn.js handler

And check that this is the case