session: refactor token store

This commit:

  * changes the set and signatures of TokenStore interface methods;

  * adds unit test for map storage.

session/store_test.go

@@ -1,3 +1,35 @@
 package session
 
-// TODO: write unit tests
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/refs"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMapTokenStore(t *testing.T) {
+	// create new private token
+	pToken, err := NewPrivateToken()
+	require.NoError(t, err)
+
+	// create map token store
+	s := NewMapTokenStore()
+
+	// create new storage key
+	id, err := refs.NewUUID()
+	require.NoError(t, err)
+
+	// ascertain that there is no record for the key
+	_, err = s.Fetch(id)
+	require.EqualError(t, err, ErrPrivateTokenNotFound.Error())
+
+	// save private token record
+	require.NoError(t, s.Store(id, pToken))
+
+	// fetch private token by the key
+	res, err := s.Fetch(id)
+	require.NoError(t, err)
+
+	// ascertain that returned token equals to initial
+	require.Equal(t, pToken, res)
+}