kselftests: cgroup: add freezer controller self-tests

This patch implements 9 tests for the freezer controller for
cgroup v2:
1) a simple test, which aims to freeze and unfreeze a cgroup with 100
processes
2) a more complicated tree test, which creates a hierarchy of cgroups,
puts some processes in some cgroups, and tries to freeze and unfreeze
different parts of the subtree
3) a forkbomb test: the test aims to freeze a forkbomb running in a
cgroup, kill all tasks in the cgroup and remove the cgroup without
the unfreezing.
4) rmdir test: the test creates two nested cgroups, freezes the parent
one, checks that the child can be successfully removed, and a new
child can be created
5) migration tests: the test checks migration of a task between
frozen cgroups: from a frozen to a running, from a running to a
frozen, and from a frozen to a frozen.
6) ptrace test: the test checks that it's possible to attach to
a process in a frozen cgroup, get some information and detach, and
the cgroup will remain frozen.
7) stopped test: the test checks that it's possible to freeze a cgroup
with a stopped task
8) ptraced test: the test checks that it's possible to freeze a cgroup
with a ptraced task
9) vfork test: the test checks that it's possible to freeze a cgroup
with a parent process waiting for the child process in vfork()

Expected output:
  $ ./test_freezer
  ok 1 test_cgfreezer_simple
  ok 2 test_cgfreezer_tree
  ok 3 test_cgfreezer_forkbomb
  ok 4 test_cgrreezer_rmdir
  ok 5 test_cgfreezer_migrate
  ok 6 test_cgfreezer_ptrace
  ok 7 test_cgfreezer_stopped
  ok 8 test_cgfreezer_ptraced
  ok 9 test_cgfreezer_vfork

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: kernel-team@fb.com
Cc: linux-kselftest@vger.kernel.org