dtc: Don't delete *.test.dtb between testgroups

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> The dtc/libfdt testsuite creates a number of .dtb files during its
> run.  To ensure a clean test run, these are currently deleted before
> each group of tests.
> 
> This is, in fact, a mistake, since if something goes wrong in the
> first group of tests, deleting the .dtb at the beginning of the second
> group of tests makes it harder to figure out what the problem was.
> 
> This patch changes the script to only delete the files once, before
> the whole test run.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl