MM-23508: Fix plugin API's GetPostsForChannel (#14125)

Add a test checking the basic behaviour of the function

app/plugin_api.go

@@ -521,7 +521,7 @@ func (api *PluginAPI) GetPostsBefore(channelId, postId string, page, perPage int
 }
 
 func (api *PluginAPI) GetPostsForChannel(channelId string, page, perPage int) (*model.PostList, *model.AppError) {
-	return api.app.GetPostsPage(model.GetPostsOptions{ChannelId: channelId, Page: perPage, PerPage: page})
+	return api.app.GetPostsPage(model.GetPostsOptions{ChannelId: channelId, Page: page, PerPage: perPage})
 }
 
 func (api *PluginAPI) UpdatePost(post *model.Post) (*model.Post, *model.AppError) {