selftests: cgroup: Add support for named v1 hierarchies in test_core

This comes useful when using selftests from mainline on older
kernels/setups that still rely on cgroup v1.
The core tests that rely on v2 specific features are skipped, the
remaining ones are adjusted to work with a v1 hierarchy.

The expected output on v1 system:
	ok 1 # SKIP test_cgcore_internal_process_constraint
	ok 2 # SKIP test_cgcore_top_down_constraint_enable
	ok 3 # SKIP test_cgcore_top_down_constraint_disable
	ok 4 # SKIP test_cgcore_no_internal_process_constraint_on_threads
	ok 5 # SKIP test_cgcore_parent_becomes_threaded
	ok 6 # SKIP test_cgcore_invalid_domain
	ok 7 # SKIP test_cgcore_populated
	ok 8 test_cgcore_proc_migration
	ok 9 test_cgcore_thread_migration
	ok 10 test_cgcore_destroy
	ok 11 test_cgcore_lesser_euid_open
	ok 12 # SKIP test_cgcore_lesser_ns_open

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>