selftests: memcg: factor out common parts of memory.{low,min} tests

The memory protection test setup and runtime is almost equal for
memory.low and memory.min cases.

It makes modification of the common parts prone to mistakes, since the
protections are similar not only in setup but also in principle, factor
the common part out.

Past exceptions between the tests:
- missing memory.min is fine (kept),
- test_memcg_low protected orphaned pagecache (adapted like
  test_memcg_min and we keep the processes of protected memory running).

The evaluation in two tests is different (OOM of allocator vs low events
of protégés), this is kept different.

Link: https://lkml.kernel.org/r/20220518161859.21565-6-mkoutny@suse.com
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
CC: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Richard Palethorpe <rpalethorpe@suse.de>
Cc: David Vernet <void@manifault.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>