[#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/api.go

@@ -0,0 +1,35 @@
+package client
+
+import (
+	"context"
+	"fmt"
+
+	v2netmap "github.com/nspcc-dev/neofs-api-go/v2/netmap"
+	rpcapi "github.com/nspcc-dev/neofs-api-go/v2/rpc"
+	"github.com/nspcc-dev/neofs-api-go/v2/rpc/client"
+)
+
+// interface of NeoFS API server. Exists for test purposes only.
+type neoFSAPIServer interface {
+	netMapSnapshot(context.Context, v2netmap.SnapshotRequest) (*v2netmap.SnapshotResponse, error)
+}
+
+// wrapper over real client connection which communicates over NeoFS API protocol.
+// Provides neoFSAPIServer for Client instances used in real applications.
+type coreServer client.Client
+
+// unifies errors of all RPC.
+func rpcErr(e error) error {
+	return fmt.Errorf("rpc failure: %w", e)
+}
+
+// executes NetmapService.NetmapSnapshot RPC declared in NeoFS API protocol
+// using underlying client.Client.
+func (x *coreServer) netMapSnapshot(ctx context.Context, req v2netmap.SnapshotRequest) (*v2netmap.SnapshotResponse, error) {
+	resp, err := rpcapi.NetMapSnapshot((*client.Client)(x), &req, client.WithContext(ctx))
+	if err != nil {
+		return nil, rpcErr(err)
+	}
+
+	return resp, nil
+}