tests/vm: give wait_ssh() option to wait for root

Allow wait_ssh to wait for root user to be ready.
This solves the issue where we perform a wait_ssh()
successfully, but the root user is not yet ready
to be logged in.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Puhov <peter.puhov@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200219163537.22098-5-robert.foley@linaro.org>
Message-Id: <20200303150622.20133-5-alex.bennee@linaro.org>