Test suite: reliably wait for post-Greeter hooks

In the I log in to a new session step we should do something which waits for all Tails Greeter post-hooks to finish so we can rid ourselves of steps like Tails Greeter has dealt with the sudo password.

This may include making changes to Tails Greeter, e.g. having it create a file when it’s done that we can wait for in the end of the I log in to a new session step.

It may be that opening a new GNOME session (which happens right after tails-greeter is done IIRC) creates such a file that we could poll for.

Feature Branch: test/5666-wait-for-greeter-PostLogin

Original created by @tails on 5666 (Redmine)