KVM: selftests: aarch64: Test read-only PT memory regions

Extend the read-only memslot tests in page_fault_test to test
read-only PT (Page table) memslots. Note that this was not allowed
before commit 406504c7 ("KVM: arm64: Fix S1PTW handling on RO
memslots") as all S1PTW faults were treated as writes which resulted
in an (unrecoverable) exception inside the guest.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230127214353.245671-5-ricarkol@google.com