[Piglit] Allow junit to be used for summary generation

[baker.dylan.c at gmail.com]

This series updates the junit backend to allow it to properly load junit
and convert it back into piglit's internal representation, thus allowing
it to be summarized using the piglit summary tools

There is still some work that needs to be done beyond this, most of the
platform metadata isn't stored yet and restored, but I have a plan for
that. I have some other refactoring work that I think will make that
easier, and I'd like to get there before landing that.

This is enough to be able to compare junit and json results using the
console and html summaries.

There is a caveat here, and that's patch 3. To compare json and junit we
need to be able to restore the names of the junit tests to *exactly*
what they were before, and currently we don't have a way to reverse the
'.' -> '_' conversion. My proposal is to change '.' into '___', which is
very unlikely in a real test name (though we could change it to almost
anything that would be unique). This may break some existing setup
(Mark, I think this will probably break some of our expected fail/crash
data).