Add a simple test for SOURCE_DATE_EPOCH
Add a simple test for SOURCE_DATE_EPOCH

This test checks if the SOURCE_DATE_EPOCH is used in the
oscap ds sds-compose command as a datastream component timestamp.

Related to https://github.com/OpenSCAP/openscap/pull/1699/

OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap