test/py/tests: Update some network dependencies

On tests which require "tftpboot" we need to depend not on cmd_net but
rather cmd_tftpboot. And on tests which require cmd_pxe we do not need
to also depend on cmd_net as this should be handled already via Kconfig
logic.

Signed-off-by: Tom Rini <trini@konsulko.com>
2 files changed