Increase unit test coverage of api/user.go (#4541)

* Add test to CheckUserDomain

* Add unit test to IsUsernameTaken

api/user.go

@@ -192,6 +192,7 @@ func createUser(c *Context, w http.ResponseWriter, r *http.Request) {
 
 }
 
+// Check that a user's email domain matches a list of space-delimited domains as a string.
 func CheckUserDomain(user *model.User, domains string) bool {
 	if len(domains) == 0 {
 		return true
@@ -1957,6 +1958,7 @@ func updateUserNotify(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// Check if the username is already used by another user. Return false if the username is invalid.
 func IsUsernameTaken(name string) bool {
 
 	if !model.IsValidUsername(name) {