tests: xml: delete comments

When building a XML tree, only one root node can be in place. This is
a "feature" added in libmxml 2.7:

<<<
mxmlLoad... did not error out on XML with multiple root nodes (Bug #403)
>>>

In libmxml 2.6 the second root node was ignored, not because it was a
comment but a bug. Our files had two root nodes, being comments or not.
libmxml accept comments, but inside the root node.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>