rpc: refactor invokecontractverify

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

pkg/rpc/server/testdata/verification_with_args_contract.go

@@ -0,0 +1,7 @@
+package testdata
+
+// Verify is a verification contract method which takes several arguments.
+func Verify(argString string, argInt int, argBool bool) bool {
+	isOK := argString == "good_string" || argInt == 5 || argBool == true
+	return isOK
+}