tests: Improve parsing coverage
tests: Improve parsing coverage

Add tests for PI erroring exactly at the buffer boundary with
non-zero-terminated buffers (so we have to clear the last character
which changes the parsing flow slightly) and a test that makes sure
parse_embed_pcdata works properly with XML fragments where PCDATA can be
at the root level but can't be embedded into the document node.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml