Test fast color clear functionality on i965/gen7+.

Fast color clears are a hardware feature in i965/gen7+ which allows
the memory writes associated with clearing a color buffer to be
deferred until the buffer is later written to or read from; in most
cases this allows the clear writes to be write-combined with later
rendering operations.

For fast color clears to work correctly, the driver must maintain a
state machine for each buffer to indicate whether there are deferred
clears pending, and if so what the clear value was.  This patch
introduces tests to validate corner cases of that state machine.