Issue title: Migrate tests from "model/saml_test.go" to use testify
 Issue body: Mattermost is migrating its server tests from using the `t.Fatal` method calls on their checks to use the [testify toolkit](https://github.com/stretchr/testify). The goal of this migration is to improve the readability of the tests, making them more concise and meaningful. This Help Wanted issue is to migrate the tests in the `model/saml_test.go` file.

The expected way to migrate this is to go to the `model/saml_test.go` file and modify the calls to `t.Fatal` and the condition checks that lead to them, replacing them with calls to the [the `require` package](https://godoc.org/github.com/stretchr/testify/require] if a failure should stop the execution of the test, or [the `assert` package|https://godoc.org/github.com/stretchr/testify/assert) if it shouldn't.

Example: mattermost/mattermost-server#12039

If you have questions about the ticket or you need any help, feel free to contact `miguel.delacruz` or `jesus.espino` in <https://community.mattermost.com/>



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JIRA: https://mattermost.atlassian.net/browse/MM-19116
    
 Comments: 
 Comment 0: I take it
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