Rename sdk-persistent-disk to sdk-persistent-disk-sysroot

This makes the test case name more explicit and keeps consistency with
the rest of the psdk tests.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

test-cases/sdk-persistent-disk.yaml → test-cases/sdk-persistent-disk-sysroot.yaml

 metadata:
-  name: sdk-persistent-disk
+  name: sdk-persistent-disk-sysroot
   format: "Apertis Test Definition 1.0"
   image-types:
     basesdk: [ amd64 ]