tests: add acceptance test for sessions play button

[rokroskar]

Currently we do not have an acceptance test for the "play" button that quick-starts interactive sessions. Recently this led to us not realizing for quite some time that the launches were not working properly for logged out users (see https://github.com/SwissDataScienceCenter/renku-ui/pull/1526). We should prevent a regression by adding an acceptance test that checks the behavior of the button in these three scenarios:

• logged in user launching a project they own or are a member of
• logged in user launching a public project they do not own
• logged out user launching a public project