tests/cursor: Set cursor theme to Adwaita

The reference images were generated using the Adwaita theme, so make sure that
the test client also draws a cursrom from the Adwaita theme rather than reading the
default theme from the environment.

Ideally the test suite would embed the cursor theme and point XCURSOR_PATH to
it, so that even if the Adwaita icon theme changes in the future the test
wouldn't suddently break; I did not implement this yet.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4225>

src/tests/wayland-test-clients/cursor-tests-client.c

@@ -127,7 +127,7 @@ on_pointer_enter (WaylandSurface    *surface,
            "and actual scale %f",
            effective_theme_size, theme_size, scale);
 
-  cursor_theme = wl_cursor_theme_load (NULL,
+  cursor_theme = wl_cursor_theme_load ("Adwaita",
                                        effective_theme_size,
                                        display->shm);
   cursor = wl_cursor_theme_get_cursor (cursor_theme, cursor_name);