tests: lib: gui: lvgl: Reorder screen load and delete

Load the default screen before the deleting the newly created one.
This used to work in LVGL 8.2.0 but was not intended according to the
discussion in zephyrproject-rtos/zephyr#53974.

Signed-off-by: Fabian Blatz <[email protected]>

Author: faxe1008