[Piglit] Add incomplete file for tests as they start

[Dylan Baker]

One of the more interesting problems for piglit, especially with
hardware bring-up where bad code may result in the system crashing, is
to know what test actually crashed the system.

This series is meant to address that problem. It does so by adding a new
status, the incomplete status, which is used by the backend to write out
a file for a test before the test starts running. When the test
completes this status is overwritten with the final result. If the test
is never overwritten (say the system crashes) it remains incomplete.