Because the OS downgrade performed in tests now triggers change in container
snapshotters, all containers need to be redownloaded. Make sure that CLI
container exists and increase the timeout for the time being.
The tests that are involving reboots are flaky and fail when waiting for the
command to return or when waiting for a new login prompt. To mitigate this, do
not use run_check, as it needs the shell prompt to reappear, and instead use
sendline and wait up to a minute for the GRUB message.
Add timeout to expect call when waiting for the OS reboot after
switching slots. While it never fails for me locally, it regularly
breaks tests in GHA.
Add test that OS update works - use the whole stack using CLI to update to the
latest stable version (unless executed manually on the latest stable release,
this version should never be the same as the currently tested one).
With this test in place, we can also test command for switching the slots, so
add an extra test for that too.
Fixes#4103
