[MM-12465] Added capability to export reactions of posts during bulk export (#9600)

* Added capability to export reactions of posts

* Added capability to export reactions of replies of a post

* Added test case to test the reactions of a post

app/export_test.go

@@ -0,0 +1,32 @@
+package app
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func TestReactionsOfPost(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+
+	post := th.BasicPost
+	post.HasReactions = true
+
+	reactionObject := model.Reaction{
+		UserId:    model.NewId(),
+		PostId:    post.Id,
+		EmojiName: "emoji",
+		CreateAt:  model.GetMillis(),
+	}
+
+	th.App.SaveReactionForPost(&reactionObject)
+	reactionsOfPost, err := th.App.BuildPostReactions(post.Id)
+
+	if err != nil {
+		t.Fatal("should have reactions")
+	}
+
+	assert.Equal(t, reactionObject.EmojiName, *(*reactionsOfPost)[0].EmojiName)
+}