Test EC policies

[roman-khimov]

With nspcc-dev/neofs-node#3570 merged we get EC policies (don't forget to enable experimental setting). We need a set of thorough tests for them. Theoretically it's at least the same set of object operation tests we do for REP containers, but with an EC twist (like expected number of objects and their exact placement where tests care). Then we will need tests for EC-specific things like object part restore (but it's in progress, nspcc-dev/neofs-node#3628).

Split objects don't work yet, see nspcc-dev/neofs-node#3607.