tests: SVGAnalyzer: save a copy of the original SVG and give access to it

tests: SVGAnalyzer: save a copy of the original SVG and give access to it

This is needed to be able to verify the SVG re-creating capablility of
the SVGAnalyzer.

Upcoming commits will extend the SVG analyzer's ability to handle new
SVG attributes and add new test cases verifying its ability to
re-create correct SVG containing those attributes. This change will
facilitate the creation of those tests and keep the code DRY.

The copying is necessary since the original SVG is modified by the XML
parser in destructive mode. See
http://rapidxml.sourceforge.net/manual.html#namespacerapidxml_1destructive_non_destructive
and we don't want to use the non-destructive mode since no entity
reference translation is done then and we want to avoid having to do
that ourselves.