tests: add tests for searchv2

[evgeniiz321]

closes #951

[roman-khimov]

PREFIX == "" is an interesting case for us to test as well.

Mixed numeric/non-numeric contents in attributes (attr=X and attr=10 in different objects).

Order of unfiltered search (which is effectively listing everything) is by OID.

[evgeniiz321]

Can't verify the case with COMMON_PREFIX == "", since CLI doesn't allow to specify empty strings. Not sure if this is by design or not.

[cthulhu-rider]

Can't verify the case with COMMON_PREFIX == "", since CLI doesn't allow to specify empty strings. Not sure if this is by design or not.

$ neofs-cli object searchv2 -h
  -f, --filters strings       Repeated filter expressions or files with protobuf JSON

JSON may help u for now. Btw if is declared, then each test should be run in both formats

[evgeniiz321]

Added test cases with empty prefix.

[evgeniiz321]

Added more cases with cursor, with/without attributes.