tests: Add pseries machine to the prom-env-test, too

Now that we also support the "-prom-env" parameter for the pseries
machine, we can enable this test for this machine, too. Since booting
with TCG is rather slow with the pseries machine, we also enable
the "-nodefaults" parameter for this test now, so that SLOF does not
have to check that much devices during boot and thus runs a little
bit faster.

Signed-off-by: Thomas Huth <thuth@redhat.com>
[dwg: Don't add -nodefaults to the command line, it causes extra warnings
 for the sparc testcases]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
