test: fit: add test case for invalid default configuration reference

test: fit: add test case for invalid default configuration reference

Recent changes to mkimage introduced a validation step to ensure that
the 'default' property under the /configurations node in a FIT image
references a valid configuration subnode. If the referenced node is
missing, mkimage will now return an error.

This patch adds a Python test case to verify that mkimage correctly
fails when the 'default' configuration does not exist. The test creates
a minimal ITS with an invalid default reference and checks that mkimage
produces the expected error message.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>