tests: Use correct path to initialize oscap_source.

Addressing:
I/O warning : failed to load external entity "data/xccdf.xml"
Introduce use of oscap_source_new_from_memory to OpenSCAP::Source.
Introduce OpenSCAP::Source.type
Allow to pass aditional info to OpenSCAP.raise
Introduce OpenSCAP::Source.validate!
Tests: OpenSCAP::Source.validate