[nrf fromtree] tests: cmake: actually test SBOM files generation

The current test was just a basic sanity check.
This adds actual validation of the SPDX files being generated as
being spec-compliant.

Signed-off-by: Benjamin Cabé <[email protected]>
(cherry picked from commit 14ea8b0)

Author: kartben

scripts/requirements-actions.in

@@ -30,6 +30,7 @@ python-magic; sys_platform != "win32"
 pyyaml
 ruff==0.11.11
 setuptools>=70.2.0
+spdx-tools
 sphinx-lint
 tabulate
 tomli>=1.1.0