Add warning for invalid "=item [text]"
suppress errors in item tests

These tests now trigger the mixed-type =item warning, new in the
preceding commit.  Rather than test for the exact error here, I have
suppressed error generation from these three specific tests only.

I think a good strategy for fixing these tests further will be to
write a JSON emitter to replace the XML one.  This will be much
easier to use in tests, as we can use JSON::PP to parse the output
into a structure.  It will make tests like this easier to update
to match errors, rather than to tweak the output to suppress them.
Merge pull request #44 from rjbs/master

Issue a warning when =item types mismatch within one =over/=back.