libfdt: Activate testcase for appending properties

Commit a31e3ef8 introduced new libfdt
functions to append to existing properties.  It also included a test case
for this, but neglected to update the Makefile and run_tests.sh script
to actually build and execute this testcase.

This patch corrects the oversight.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>