tests/xts.py: Add a tests file for the X Test suite.

This adds support for running XTS from piglit.

This patch was original written by Eric Anholt, but I've cleaned it up
and reworked parts of it for upstreaming, and as a result I've attached
my name as the author so ``git bisect'' and ``git blame'' will send
angry mail my way instead of Eric's.

v2: - Don't break dmesg reporting
    - Use str.format consistently
    - populate profile after check for xtest symlink

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>