Changes

node: Fix dom_node_normalize (via _dom_merge_adjacent_text)
Normalizing is meant to turn a sequence of text nodes into one text node
representing the sequence; unfortunately due to the way that
_dom_merge_adjacent_text had been written this actually resulted in an
exponential repetition of the text content.
This is fixed by using the underlying characterdata objects instead.
Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

test: Add a normalize test alongside the XML tests
Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>