selftests: sync: use TEST_CUSTOM_PROGS instead of TEST_PROGS

lib.mk var TEST_CUSTOM_PROGS is for tests that need custom build
rules. TEST_PROGS is used for test shell scripts. Fix it to use
TEST_CUSTOM_PROGS. lib.mk will run and install them.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>