Refactor: libcrmcommon: Don't reuse doc in pcmk__xc_create() unit test

If we simply assume that pcmk__xml_new_doc() returns non-NULL (as it
currently must because it asserts on allocation error), then it's simple
to make assert_comment() more self-contained.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>