feat: parse and import XCCDF with root-level TestResult

[csmig]

This PR synchronizes XCCDF parsing code between the STIG Manager web app and Watcher. See this STIG Manager pull request.

This code successfully processes the sanitized sample files from ACAS and HBSS which were provided with here. We hope for 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.