In `(c *Client) AddNetworkFee` we define network fee for contract
witness verification via `invokecontractverify` RPC call, and that's the
initial purpose of this RPC method. But it was not implemented
correctly. It used `System.Contract.Call` instead of beheiving like
`initVerificationVM`.
During real contract witness verification the whole contract's script is
loaded into VM, and then we jump to the `verify` method. Thus, to define
exact contract verification price, we should act like this (and not just
perform `System.Contract.Call` of `verify` method).
Tests are added.
This bug is the reason of adding extra GAS (c.notary.extraVerifyFee) to
pre-calculated value in
https://github.com/nspcc-dev/neofs-node/pull/404/files#diff-639db437ca2578db46c9e8cbf18f9aa01f8ca5aee30e0fa7e70ba0354822d7b3R237
