test: Wait for user sessions to work before using a test machine
test: Wait for user sessions to work before using a test machine

Login as non-root only works after the systemd-user-sessions.service
has started on Fedora, for example.  We race against this when
starting the tests immediately after the network is up.

So we wait until the "login:" prompt appears in the boot output.

Fixes #1654
Closes #1661
Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>

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