XML: upgrade-2.10-roundtrip.xsl et al.: new tests incl. browser view

As already mentioned in this new template "compositor":

For experimenting and maintenance purposes only, pacemaker shall
split the transformation pipeline on its own.

(which is already the case).

Note that upgrade-2.10-htmldiff.xsl is switched to use such a superset
transformation so the browser based test drive (regression.sh -W) works
as expected also for new test2to3roundtrip suite.