Add conformance tests

[Mingun]

The official test suite is located at https://www.w3.org/XML/Test/. The all links from that page or found by references from that page:

• Links under Releases header. Only last test-suite we should use (this is different versions of the same data)
• direct link to the latest test-suite: https://www.w3.org/XML/Test/xmlts20130923.zip
• CVS repository: https://dev.w3.org/cvsweb/2001/XML-Test-Suite/
• Java generator: https://github.com/belingueres/xml-conformance
• NPM package: https://github.com/lddubeau/xml-conformance-suite

There also exist W3C XML Schema 1.1 (XSD 1.1) test suite, maybe would be useful: https://github.com/w3c/xsdtests

The preferred implementation should automatically download test suite if it does not exist locally and run tests. We also could have a copy as a submodule in our repository. I prefer to use default Rust tools, i.e. cargo test. Test could be implemented in a separated crate to not slow down everyday workflow.