DPU Tests not working with FilesDataUnit

[jakubklimek]

I cannot write a DPU test for DPU which works with FilesDataUnit. Since XSLT no longer supports passing input XML document as RDF literal, I had to rewrite my DPU to put the XML in a file in FilesDataUnit. There is some problem but I cannot debug it without tests.

[tomas-knap]

I agree, it is necessary to provide debugging environments for FILES data unit as it exists for RDF data unit

Should be resolved in develop branch, test.