feat: parse and import XCCDF with root-level TestResult

[csmig]

Resolves: #890

This code successfully processes the sanitized sample files from ACAS and HBSS which were provided with #890. We need additional feedback from the community to establish we can handle all of the real-world XCCDF from those tools.

This PR handles the non-standard value for <TestResult test-system> that is seen in the HBSS file. According to the XCCDF specification the test-system attribute "SHOULD be either a CPE name or a CPE applicability language expression", but HBSS does not follow this suggestion and our existing code expected a CPE name.

Unlike SCC and OpenSCAP, neither ACAS nor HBSS provide a timestamp for each <rule-result>. I've instead used the required attribute <TestResult end-time> as a common timestamp for each result.

[ronaldsimmons]

@csmig Thank you, all I need the Cloud One team to do now is do a pull