efi_selftest: SNP unit test on sandbox

Running the simple network protocol test on the sandbox requires setting
the environment variable ethact to a network interface connected to a DHCP
server and ethrotate to 'no'. So let's make it an on-request test on the
sandbox (selectable by setting environment variable efi_selftest).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>