Test+CI: finally care to update stale links to "net resource" schemas

This enables reverting of 90f5c2fd6, which is likewise done here.

Some interesting circumstances:

• relaxng.org content is now served from GitHub, yielding: https://github.com/relaxng/relaxng.org (the date of the first commit there, 2018-11-01, coincides approximately with when the problems where spotted, i.e., a month later)
• the main mover behind that, Norman Walsh, also maintains XSLT schemas in RelaxNG Compact form: https://github.com/ndw/xslt-relax-ng (RNC is convertible to RNG using trang)
• the above was of interest since we originaly consumed xslt.rng also as a network resouece, from thaiopensource.com domain, which also appears to be served from GitHub, this time, it would have something to do with https://github.com/relaxng/relaxng.org/tree/master/jclark, but alas, xslt.rng is mising there
• fortunately, xslt.rng is present directly in jing-trang project (along with relaxng.rng, but feels more natural to consume it from a somewhat authoritative source instead in that case, see above) https://github.com/relaxng/jing-trang/tree/master/eg
• libxml2 is dumb when it comes to fetching network resources over HTTPS, so we play the already applied trick with XML_CATALOG_FILES (see 2f6e74883) to avoid unnecessary churn with xmllint, especially since jing accepts those URLs just fine (IIRC) and we want to retain that flexbility