drm/tests: Test drm_fb_build_fourcc_list() in separate test suite

Only sysfb drivers use drm_fb_build_fourcc_list(). The helper will
be moved from format helpers to sysfb helpers. Moving the related
tests to their own test suite.

v3:
- rename tests according to filename (José)
v2:
- rename filename to match tested code (Maxime)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250616083846.221396-3-tzimmermann@suse.de