Introduce: OpenSCAP::Xccdf::TestResult.source
Rename TestResult FFI::Pointer to raw

It turns out that it is nice patter to keep all the FFI::Pointers
of wrapper classes named raw.
Introduce: OpenSCAP::Xccdf::TestResult.raw
Allow the update of test_result within the ARF XML.
OpenSCAP::DS::Arf.source shall be accessible from outside

So we don't need to implement save dupe method for ARF.
tests: Load the ARF once again.