Restructure testing

[lodo1995]

• Move the current testing code from unittests to a separate testing module
• Include the W3C XML Test Suite from https://www.w3.org/XML/Test/
• Include BOM handling, so that the Test Suite can be used

[lodo1995]

To be used completely, the XML Test Suite requires the ability to parse DTDs and external entities, so for now only part of it will be used in testing (even if I will copy all of it in the project). For this reason, DTD parsing and entity substitution (at least for internal entities) gains a higher priority than before.

[burner]

for parsing that is true, but for lexing aka split the string into xml nodes all files should work. most likely

[lodo1995]

Closing; now the structure for testing is set up.