test/py: Handle expected reboot while booting sandbox

Add expected_reset optional argument to ConsoleBase::ensure_spawned(),
ConsoleBase::restart_uboot() and ConsoleSandbox::restart_uboot_with_flags()
so that it can handle a reset while the 1st boot process after main
boot logo before prompt correctly.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>