Simple browser tests

[djay]

Should be possible to do a simple browser test

• setup 2 plone sites (or maybe just 2 PAS)
• config 1 as IdP and 1 as SP
• get login page on SP
• fake the redirect to IdP since that is JS (do submit)
• fake the redirect back to SP since that is JS? (do submit)
• see if the user logged to the SP

[djay]

Started a PR in #11 but got a bit stuck trying to have two different test plone sites in the same test layer