test: Add test case for FIT image load address overlap detection

test: Add test case for FIT image load address overlap detection

Add a new test case to verify that mkimage properly detects and reports
memory region overlaps in FIT image configurations.

The test creates a FIT image with kernel and FDT components that have
the same load address (0x40000), which should trigger the overlap
detection logic and cause mkimage to fail with an appropriate error
message.

Test verifies:
- mkimage returns non-zero exit code when overlap is detected
- Error message contains "Error: Overlap detected:"
- Error message identifies the specific overlapping components
  (kernel@1 and fdt@1)

This test ensures the overlap detection feature works correctly and
prevents deployment of FIT images with conflicting memory layouts
that could cause runtime failures.

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