We only had to pile everything into the src/fuzzing/firmware directory
because honggfuzz could not cope with more than one input path.
This way each plugin is self contained and easy to copy.
Also, install the fuzzing builder objects as this fixes the installed
tests when srcdir does not exist.
Based on a patch by Jan Tojnar <jtojnar@gmail.com>, many thanks.
This allows us to override the location we load data files from, which
allows us to do more kinds of installed tests in the future.
Also, move the global data/tests content into the place that it is used
as it was getting impossible to manage.
