dtc: Cleanup testsuite organization

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> run_tests.sh from the dtc testsuite currently has a facility ro run
> just "functional" or just "stress" tests.  This distinction is carried
> over from libhugetlbfs where the test framework originated, and where
> it made sense.
> 
> In dtc, we have no stress tests, so running these subsections isn't
> particularly interesting.  This patch removes these test subsets,
> instead defining a single "libfdt" test subset for testcases related
> to libfdt (and not dtc proper only.  Currently that's all of the
> testcases, but with any luck we'll have some dtc testcases in the
> future.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl