saml: add tests case covering tampered NameID field (comment)

As sketched here:

https://developer.okta.com/blog/2018/02/27/a-breakdown-of-the-new-saml-authentication-bypass-vulnerability



Thought it was interesting to see how our SAML connector behaved. And
it seems to be behaving well. :)

Signed-off-by: Stephan Renatus <srenatus@chef.io>