Covering Units Tests for XML Stylesheets from #63

[craftyshaun]

I'd love to see some unit tests validating behaviour but that's up to you. Could be merged w/o it.

I was planning to add these covering tests as part of v3 or as an additional issue post-merge. I'll create an issue for the example sitemap and further tests (assuming it is not going to be re-written in 3.x)

Originally posted by @craftyshaun in https://github.com/samdark/sitemap/issues/63#issuecomment-929686305