[xmlroff] make testing more accessible to contributors

[Stefan S. <se...@sy...>]

When trying to confine a bug I tried to first see whether it was already
covered by an existing test. Unfortunately, I found the existing test
infrastructure a bit confusing / hard to use.

Is there anything that could be done to make that easier ? Ideally
I would only need to have to check out an additional module (not two !)
and then run 'make check' (or similar) in it.
I did glance over some READMEs both in 'testsuite' as well as 'testing',
but couldn't quite get it to work.

Thanks,
		Stefan