Added OVAL 5.11 Release Candidate schemas
Resolve parsing issues with OVAL 5.11 schemas

Similar to e64ba09361ab2e62125abc38b96b8918f78b1fac but for OVAL 5.11 schemas
Enabled validation for systemd tests, fixed them to pass the validation
Added OVAL 5.11 schemas to Makefile.am
Use the shared xmldsig-core-schema.xsd even in OVAL 5.11 schemas