Unittests: Add unittests for HTML output

Provides a set of static files to perform basic unittesting against.
unittest: Do not use same location for all tests

Previous behavior had each suite replacing the previous suite's files,
which may be efficient for space, but for both debugging reasons and
robustness reasons doesn't make a lot of sense. This patch corrects that
behavior by using /tmp/ut as a root folder, and each suite adds a new
folder under that.
unittests: Adds test for junit xml that validates using xmllint

This test currently only runs on linux (since it relies on linux
binaries xmllint and which). It grabs the latest copy of junit-4.xsd for
the jenkins upstream, and uses xmllint and that file to verify the xml
that piglit-summary-junit.py creates.
unittests: Adds testsuite for HTML output with broken json file

This exercises the path that takes the summary though the rebuilding of
the json file.
piglit-unit-test.py: Adds some commments
unittest: Marks junit xmllint test as expected failure

The current implementation of junit output is not compliant, since I'm
waiting to merge new junit generation code I have not interest in fixing
this, so it's easier to just mark it as broken.