piglit-test.py: use full concurrency

This flips the always run everything concurrently switch on. This is
safe when using wayland and gbm (gbm being currently hard coded on), and
is safe on X11 if there is a compositor enabled (gnome-shell, kwin,
etc). This can make test runs considerably faster.
mesa/build.py: enable debug builds

Add the --enable-debug flag to mesa when build.py is invoked with
--config debug
Merge pull request #4 from dcbaker/enable-debug

mesa/build.py: enable debug builds
build_specification.xml: enable g965 testing