core: add System.Binary.Base64Encode(Decode) interops

Part of #1055

pkg/core/interop_neo_test.go

@@ -1,6 +1,7 @@
 package core
 
 import (
+	"encoding/base64"
 	"fmt"
 	"testing"
 
@@ -214,6 +215,39 @@ func TestECDSAVerify(t *testing.T) {
 	})
 }
 
+func TestRuntimeEncode(t *testing.T) {
+	str := []byte("my pretty string")
+	v, ic, bc := createVM(t)
+	defer bc.Close()
+
+	v.Estack().PushVal(str)
+	require.NoError(t, runtimeEncode(ic, v))
+
+	expected := []byte(base64.StdEncoding.EncodeToString(str))
+	actual := v.Estack().Pop().Bytes()
+	require.Equal(t, expected, actual)
+}
+
+func TestRuntimeDecode(t *testing.T) {
+	expected := []byte("my pretty string")
+	str := base64.StdEncoding.EncodeToString(expected)
+	v, ic, bc := createVM(t)
+	defer bc.Close()
+
+	t.Run("positive", func(t *testing.T) {
+		v.Estack().PushVal(str)
+		require.NoError(t, runtimeDecode(ic, v))
+
+		actual := v.Estack().Pop().Bytes()
+		require.Equal(t, expected, actual)
+	})
+
+	t.Run("error", func(t *testing.T) {
+		v.Estack().PushVal(str + "%")
+		require.Error(t, runtimeDecode(ic, v))
+	})
+}
+
 // Helper functions to create VM, InteropContext, TX, Account, Contract.
 
 func createVM(t *testing.T) (*vm.VM, *interop.Context, *Blockchain) {