oe-selftest: fitimage: test absent optional nodes in ITS files

oe-selftest: fitimage: test absent optional nodes in ITS files

Extend the test framework to verify that certain optional nodes are properly
absent from ITS files based on configuration. The _get_req_its_paths()
method now returns a tuple containing both expected and not-expected
paths, enabling negative testing of conditional components.

Test improvements:
- Add verification for absent bootscr, setup, and ramdisk image nodes
  when their respective features are disabled
- Extend configuration node testing with proper kernel/fdt/ramdisk
  field validation based on device tree and initramfs settings

Code cleanup:
- Remove unused tempfile module import
- Sort bb_vars keys alphabetically in _test_fitimage_py()
- Add debug output for bb_vars overrides when debug logging is enabled
- Remove trailing empty line
- Fix DTB file ordering for consistent test results

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>