kleaf: add integration tests for kernel_config_devicetree_library

Adds integration tests in the DDK workspace setup to verify
end-to-end compilation of a .dts file using the new macro
against both source and prebuilt kernels, asserting that
CONFIG_MODULES is successfully read.

TAG=agy
CONV=6725acaa-958d-41cb-bca4-dd11d5319381

Bug: 448483795
Test: tools/bazel run //build/kernel/kleaf/tests/integration_test -- DdkWorkspaceSetupTest.test_setup_with_local_prebuilts
Assisted-by: Antigravity:Gemini-Next
Change-Id: Iba161821c8be4670cf1bb8728b1f3a42ccf12aaa