patchtest: sort when reading patches from a directory

patchtest: sort when reading patches from a directory

When reading patches from a directory it's important to sort the output
of os.listdir(), as that returns the files in an effectively random
order.  We can't test the patches apply if they're applied in the wrong
order, and typically patch filenames are prefixed with a counter to
ensure the order is correct.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>