1. Allow 1-block slippage for multisignature request block generation to
ensure that PostPersist for fb's NVB-th block is properly finished.
2. Allow 1-block slippage before sent request checks to ensure that
PostPersist for NVB fb's NVB-th block was finished and all
stale mains were properly marked by Notary service.
Overall, this commits ports the approach of #3390 to the whole test.
Close#3366.
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>