Update sdk-fullscreen testcase

remove deprecated pkg `Xephyr` and use agl-compositor

Signed-off-by: Chinthalapudi Siva Krishna Prasad <sivakrishnaprasad.chinthalapudi@in.bosch.com>


(cherry picked from commit d27919af)

test-cases/sdk-vb-fullscreen.yaml

@@ -39,6 +39,6 @@ run:
     - "Start the SDK virtual machine"
     - "Click \"Switch to Fullscreen\" from the \"View\" menu"
     - "Open a terminal"
-    - "Launch a program using GLX under Xephyr:"
-    - $ Xephyr :1 -host-cursor -screen 800x480x24 -ac &
+    - "Run below commands"
+    - $ agl-compositor
     - $ DISPLAY=:1 /usr/lib/x86_64-linux-gnu/webkit2gtk-4.0/MiniBrowser webkit2gtk/webgl-square.html