Golang Unit Testing

I am stuck on how to write tests. I have following public function. GetAccountNumber() has its own logic. I would like to return a mock value for GetAccountNumber(). In Java, I could do when(object.method()).thenReturn("returnValue"). Is something similar available for go how can I accomplish that. I am going through bunch of go documentation but have not found anything useful yet.

func GetConfigName() string {
    var configName string
    accountNumber := GetAccountNumber()
    switch *accountNumber {
    case "123":
        configName = "test1"
    case "456":
        configName = "test2"

    return configName
}