PLT-6474: Server: Add elasticsearch/test endpoint to API. (#6792)

einterfaces/elasticsearch.go

@@ -10,6 +10,7 @@ type ElasticSearchInterface interface {
 	IndexPost(post *model.Post, teamId string)
 	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams) ([]string, *model.AppError)
 	DeletePost(postId string)
+	TestConfig() *model.AppError
 }
 
 var theElasticSearchInterface ElasticSearchInterface