tests/boot-serial: Enable the boot-serial test on SPARC machines, too

OpenBIOS prints out the name of the detected CPU here, so looking for
this string is a nice test to verify that the CPU detection is still
working correctly.

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>