Add test for publisher spool encode and decode. (elastic#15534)

* Add test for publisher queue encode and decode.

* Run mage fmt.

* Fixes from code review.

libbeat/publisher/queue/spool/codec_test.go

@@ -0,0 +1,76 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package spool
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/elastic/beats/libbeat/beat"
+	"github.com/elastic/beats/libbeat/common"
+	"github.com/elastic/beats/libbeat/publisher"
+)
+
+func TestEncodeDecode(t *testing.T) {
+	tests := map[string]codecID{
+		"json":   codecJSON,
+		"ubjson": codecUBJSON,
+		"cborl":  codecCBORL,
+	}
+
+	fieldTimeStr := "2020-01-14T20:33:23.779Z"
+	fieldTime, _ := time.Parse(time.RFC3339Nano, fieldTimeStr)
+	event := publisher.Event{
+		Content: beat.Event{
+			Timestamp: time.Now().Round(0),
+			Fields: common.MapStr{
+				"time":       fieldTime,
+				"commontime": common.Time(fieldTime),
+			},
+		},
+	}
+	expected := publisher.Event{
+		Content: beat.Event{
+			Timestamp: event.Content.Timestamp,
+			Fields: common.MapStr{
+				"time":       fieldTime.Format(time.RFC3339Nano),
+				"commontime": common.Time(fieldTime).String(),
+			},
+		},
+	}
+
+	for name, codec := range tests {
+		t.Run(name, func(t *testing.T) {
+			encoder, err := newEncoder(codec)
+			assert.NoError(t, err)
+
+			encoded, err := encoder.encode(&event)
+			assert.NoError(t, err)
+
+			decoder := newDecoder()
+			decoder.buf = encoded
+
+			observed, err := decoder.Decode()
+			assert.NoError(t, err)
+
+			assert.Equal(t, expected, observed)
+		})
+	}
+}