[#270] client/netmap: Cover NetMapSnapshot with unit tests

There is a need to test each `Client` operation. In previous
implementation `Client` was based on real socket connection. This didn't
allow to test the `Client` without OS resources. In order to write
convenient and useful unit tests we need to slightly refactor the code.

Introduce `neoFSAPIServer` interface as a provider of `Client` type's
abstraction from the exact NeoFS API server. Add `netMapSnapshot` method
for initial implementation. Define core interface provider used in real
code. Set `coreServer` as an underlying `neoFSAPIServer` in
`Client.Dial`. Cover `Client.NetMapSnapshot` method with unit tests
using the opportunity to override the server.

From now client library can be tested not only with real physical
listeners but with imitations.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/client_test.go

@@ -0,0 +1,39 @@
+package client
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"testing"
+
+	apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+	"github.com/stretchr/testify/require"
+)
+
+/*
+File contains common functionality used for client package testing.
+*/
+
+var key, _ = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+
+var statusErr apistatus.ServerInternal
+
+func init() {
+	statusErr.SetMessage("test status error")
+}
+
+func assertStatusErr(tb testing.TB, res interface{ Status() apistatus.Status }) {
+	require.IsType(tb, &statusErr, res.Status())
+	require.Equal(tb, statusErr.Message(), res.Status().(*apistatus.ServerInternal).Message())
+}
+
+func newClient(server neoFSAPIServer) *Client {
+	var prm PrmInit
+	prm.SetDefaultPrivateKey(*key)
+
+	var c Client
+	c.Init(prm)
+	c.setNeoFSAPIServer(server)
+
+	return &c
+}