[MM-14779] Investigate flaky TestExportGMandDMChannels (#10534)

* fix flaky test by sorting by Type and also change helper to increase
precision in CreateAt

* better strategy is to get a smaller number

* user nanonseconds as milisenconds proved to be a bit problematic for
unit tests

* revert changes in helper function and create posts in a more controled way

* use the original value

* make post instantiation more clear

app/export_test.go

@@ -354,7 +354,7 @@ func TestExportGMandDMChannels(t *testing.T) {
 	channels = result.Data.([]*model.DirectChannelForExport)
 
 	// Adding some deteminism so its possible to assert on slice index
-	sort.Slice(channels, func(i, j int) bool { return channels[i].CreateAt > channels[j].CreateAt })
+	sort.Slice(channels, func(i, j int) bool { return channels[i].Type > channels[j].Type })
 	assert.Equal(t, 2, len(channels))
 	assert.ElementsMatch(t, []string{th1.BasicUser.Username, user1.Username, user2.Username}, *channels[0].Members)
 	assert.ElementsMatch(t, []string{th1.BasicUser.Username, th1.BasicUser2.Username}, *channels[1].Members)
@@ -377,11 +377,34 @@ func TestExportDMandGMPost(t *testing.T) {
 	gmMembers := []string{th1.BasicUser.Username, user1.Username, user2.Username}
 
 	// DM posts
-	th1.CreatePost(dmChannel)
-	th1.CreatePost(dmChannel)
+	p1 := &model.Post{
+		ChannelId: dmChannel.Id,
+		Message:   "aa" + model.NewId() + "a",
+		UserId:    th1.BasicUser.Id,
+	}
+	th1.App.CreatePost(p1, dmChannel, false)
+
+	p2 := &model.Post{
+		ChannelId: dmChannel.Id,
+		Message:   "bb" + model.NewId() + "a",
+		UserId:    th1.BasicUser.Id,
+	}
+	th1.App.CreatePost(p2, dmChannel, false)
+
 	// GM posts
-	th1.CreatePost(gmChannel)
-	th1.CreatePost(gmChannel)
+	p3 := &model.Post{
+		ChannelId: gmChannel.Id,
+		Message:   "cc" + model.NewId() + "a",
+		UserId:    th1.BasicUser.Id,
+	}
+	th1.App.CreatePost(p3, gmChannel, false)
+
+	p4 := &model.Post{
+		ChannelId: gmChannel.Id,
+		Message:   "dd" + model.NewId() + "a",
+		UserId:    th1.BasicUser.Id,
+	}
+	th1.App.CreatePost(p4, gmChannel, false)
 
 	result := <-th1.App.Srv.Store.Post().GetDirectPostParentsForExportAfter(1000, "0000000")
 	posts := result.Data.([]*model.DirectPostForExport)
@@ -409,7 +432,7 @@ func TestExportDMandGMPost(t *testing.T) {
 	posts = result.Data.([]*model.DirectPostForExport)
 
 	// Adding some deteminism so its possible to assert on slice index
-	sort.Slice(posts, func(i, j int) bool { return posts[i].CreateAt > posts[j].CreateAt })
+	sort.Slice(posts, func(i, j int) bool { return posts[i].Message > posts[j].Message })
 	assert.Equal(t, 4, len(posts))
 	assert.ElementsMatch(t, gmMembers, *posts[0].ChannelMembers)
 	assert.ElementsMatch(t, gmMembers, *posts[1].ChannelMembers)