tests/avocado: apply a band aid to aspeed-evb login

tests/avocado: apply a band aid to aspeed-evb login

This is really a limitation of the underlying console code which
doesn't allow us to detect the login: and following "#" prompts
because it reads input line wise. By adding a small delay we ensure
that the login prompt has appeared so we don't accidentally spaff the
shell commands to a confused getty in the guest.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Acked-by: John Snow <jsnow@redhat.com>
Message-Id: <20220811151413.3350684-8-alex.bennee@linaro.org>