tests/boot-serial-test: fix powernv support

Recent commit introduced the firmware image skiboot 5.9 which
has a different first line ouput.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

tests/boot-serial-test.c

@@ -72,7 +72,7 @@ static testdef_t tests[] = {
     { "ppc64", "ppce500", "", "U-Boot" },
     { "ppc64", "prep", "", "Open Hack'Ware BIOS" },
     { "ppc64", "pseries", "", "Open Firmware" },
-    { "ppc64", "powernv", "-cpu POWER8", "SkiBoot" },
+    { "ppc64", "powernv", "-cpu POWER8", "OPAL" },
     { "i386", "isapc", "-cpu qemu32 -device sga", "SGABIOS" },
     { "i386", "pc", "-device sga", "SGABIOS" },
     { "i386", "q35", "-device sga", "SGABIOS" },