Add a test case to verify that mkimage correctly rejects a FIT source
that references a non-existent image from a configuration node.
This test introduces a minimal ITS that defines a valid kernel image
but references a missing "fdt" image under the /configurations section.
The test asserts that mkimage fails with a clear error message, as
introduced in the new validation logic.
This helps ensure the validation logic behaves correctly and prevents
regressions in future FIT enhancements.
Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
