test: add large pattern test

test: add large pattern test

Add a test which writes a file with a 16KB buffer pattern, then
verify the file with the same pattern.

The test writes a single 16KB block and thus should be the same as
the patttern file. Verify that this is the case after the test is
run.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>