XML: regression.sh: fix explicit test suite selection

Also, rename s/test_2to3/test2to3/ (shorter is better, also for
the purpose of enumerating available tests in the help screen),
and parametrize RelaxNG validator (provided it satisfies the
declared interface -- adaptor may be needed for new ones).

Also honour that /dev/null will not be positively matched with
"test -f", as opposed to "test -e", rectify "./regression -"
case (read the test selection from stdin), and fix possibly
undefined variable being used due to "read" encountering
end-of-file or some such.

Last but not least, stop triggering the "clean" phase when
referential outputs get generated.