test_list is an iterator, you can't walk an iterator more than once. To
solve this we use itertools.tee, which creates a shared buffer for the
iterators to draw from. This fixes errors like:

[000/480]
Traceback (most recent call last):
  File "./piglit", line 178, in <module>
    main()
  File "./piglit", line 174, in main
    sys.exit(runner(args))
  File "/home/user/piglit/framework/exceptions.py", line 51, in _inner
    func(*args, **kwargs)
  File "/home/user/piglit/framework/programs/run.py", line 370, in run
    backend.finalize({'time_elapsed': time_elapsed.to_json()})
  File "/home/user/piglit/framework/backends/json.py", line 163, in finalize
    assert data['tests']
  AssertionError

Thanks Tomi for figuring out what was wrong with the original patch!

CC: Tomi Sarvela <tomi.p.sarvela@intel.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>