scripts/sbom: add unit tests for SPDX-License-Identifier parsing

scripts/sbom: add unit tests for SPDX-License-Identifier parsing

Verify that SPDX-License-Identifier headers at the top of source files
are parsed correctly.

Assisted-by: Cursor:claude-sonnet-4-5
Assisted-by: OpenCode:GLM-4-7
Co-developed-by: Maximilian Huber <maximilian.huber@tngtech.com>
Signed-off-by: Maximilian Huber <maximilian.huber@tngtech.com>
Signed-off-by: Luis Augenstein <luis.augenstein@tngtech.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>