Tests are added for the PRONOM types work along with new helper
functions for making Siegfried tests more discrete and maintainable.

PRONOM identifier related linting fixes for the different source
files touched by the PRONOM types additions.

We can avoid writing to disk and make the tests here more portable by
reading from an in-memory filesystem. The skeletons themselves are
small and so can be easily stored in-line as strings and then turned
into byte objects.

Given the refactor to in-memory objects, we also take the opportunity
to add a file that won't identify with the minimal PRONOM signature
file and PRONOM reports. Type should be a nil-string as with many
of the other fields.

Ensures that the DROID header doesn't change in code unless it
is explicitly made to do so.