MM-11521/MM-11522 Fix being able to create users with invalid emails through API (#9199)

* MM-11522 Fix being able to create users with invalid emails through API

* Ensure store tests are using valid emails

* Add missing license header

* Remove invalid test case

* Fix TestUpdateOAuthUserAttrs

store/storetest/storetestlib.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package storetest
+
+import (
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func MakeEmail() string {
+	return "success_" + model.NewId() + "@simulator.amazonses.com"
+}