Remove LDAP save test

This is handled within the LDAP class
Add regressiontest to verify allow_single_sign_on setting

verification for #1677

Since testing omniauth_callback_controller.rb is very difficult, the logic
is moved to the models
Merge branch 'issue-respect-single-sign-on' into 'master'

Issue respect single sign on

Adds regression test for #1677

See merge request !1188