Move test_dev_cgroup to prog_tests to be able to run it with test_progs.
Replace dev_cgroup.bpf.o with skel header file, dev_cgroup.skel.h and
load program from it accourdingly.

  ./test_progs -t test_dev_cgroup
  mknod: /tmp/test_dev_cgroup_null: Operation not permitted
  64+0 records in
  64+0 records out
  32768 bytes (33 kB, 32 KiB) copied, 0.000856684 s, 38.2 MB/s
  dd: failed to open '/dev/full': Operation not permitted
  dd: failed to open '/dev/random': Operation not permitted
  #365     test_dev_cgroup:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
Changes since v1:
- Rename file from test_dev_cgroup.c to dev_cgroup.c
- Use ASSERT_* in-place of CHECK