[Intel-gfx] Changes to running intel-gpu-tools with Piglit

[Thomas Wood]

I've just updated the instructions in the intel-gpu-tools README to
detail the new ways to run the tests.

The most important change is that the igt symlink support has been
removed from Piglit. Anyone using the symlink to tell Piglit where to
locate the tests needs to update to either use piglit.conf or set the
IGT_TEST_ROOT environment variable when running Piglit.

There is also a new script (scripts/run-tests.sh) to help run the test
suite. This takes care of setting common Piglit options and can also
be used to download Piglit locally if it is not already installed. The
script has the same include and exclude filters as Piglit (-t and -x)
and can also immediately produce the HTML summary after the test run
if used with -s.

Regards,

Thomas