Basic tests of 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 simple shader_runner tests to validate corner cases of that
state machine.
Test reading from a buffer after fast clearing it.