Merge branch 'selftests/bpf: convert three other cgroup tests to test_progs'

Alexis Lothoré (eBPF Foundation) says:

====================
Hello,
this series brings a new set of test converted to the test_progs framework.
Since the tests are quite small, I chose to group three tests conversion in
the same series, but feel free to let me know if I should keep one series
per test. The series focuses on cgroup testing and converts the following
tests:
- get_cgroup_id_user
- cgroup_storage
- test_skb_cgroup_id_user

Changes in v4:
- Fix test after netns addition by making sure loopack interface is up
- Link to v3: https://lore.kernel.org/r/20240812-convert_cgroup_tests-v3-0-47ac6ce4e88b@bootlin.com

Changes in v3:
- Fixed multiple leaks on cgroup file descriptors and sockets
- Used dedicated network namespaces for tests involving network
- Link to v2: https://lore.kernel.org/r/20240806-convert_cgroup_tests-v2-0-180c57e5b710@bootlin.com

Changes in v2:
- Use global variables instead of maps when possible
- Collect review tags from Alan
- Link to v1: https://lore.kernel.org/r/20240731-convert_cgroup_tests-v1-0-14cbc51b6947@bootlin.com
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>