selftests: KVM: Rename psci_cpu_on_test to psci_test

There are other interactions with PSCI worth testing; rename the PSCI
test to make it more generic.

No functional change intended.

Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220504032446.4133305-9-oupton@google.com