XSLT conformance tests suite is now open for business

This is just a silly informational "issue", so that people can find it.

We can now run the XSLT conformance tests; it's described in hacking and works very similarly to running the XPath ones. The test runner for XSLT misses a bunch of XSLT specific features, so if you have failing tests also take a look at what the conformance tests is (not) doing. We will improve it incrementally.

Right now there are likely a LOT of low hanging fruits; making hundreds or even thousands of new tests pass with a few commits is likely possible, if my experience the XPath conformance test suite applies here.