tests: add store dword tests

tests: add store dword tests

Add a couple of simple store dword tests to test memory coherence.

gem_storedw_loop simply executes a batch that continually stores an
incremented value to a target buffer object, checking the results after
each batch completes.

gem_storedw_batches_loop does the same thing, but creates a new command
batch buffer for each iteration, which can exercise the buffer creation
code.  This test is based on one from Andrzej Kacprowski from Intel.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>