test: py: add mkimage test for undefined image references in FIT configs

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>

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