dtc: Add testcases exercising -Odts mode

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch adds a batch of testcases exercising dtc's -Odts mode.
> Specifically it checks that using dtc to convert dtb->dts->dtb
> preserves the original dtb for a number of example dtb files.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl