Fix Makefile to set safesetid-test.sh to TEST_PROGS instead
of non existing run_tests.sh.

Without this fix, I got following error.
  ----
  TAP version 13
  1..1
  # selftests: safesetid: run_tests.sh
  # Warning: file run_tests.sh is missing!
  not ok 1 selftests: safesetid: run_tests.sh
  ----

Fixes: c67e8ec0 ("LSM: SafeSetID: add selftest")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>