Refactor of the tests directory

Hi folks,

In the course of testing my implementation (now passing 171 of 173
tests; one I think should pass and one I think requires discussion[1]),
I reworked the tests directory completely:

  https://github.com/invisibleXML/ixml/pull/32

I hope that the changes are acceptable. I think it would be useful to
have a repository of discrete tests about which we can report
conformance and raise issues.

                                        Be seeing you,
                                          norm

[1] https://github.com/invisibleXML/ixml/issues/31

--
Norm Tovey-Walsh
Saxonica