XML: regression.sh: allow for deblanked validation of outcomes

This is to prevent subtle issues when the input to XSLT doesn't contain
any (ignorable) blanks, which is exactly what happens in pacemaker's
XML handling for XML_PARSE_NOBLANKS being employed when using
libxml2 parser.

[using time-travel here, as in one of the commits, there was an issue
observed with "normalize-space($SomeTextSelect) = $SomeTextSelect"
that, surprisingly, doesn't hold when "$SomeTextSelect" matched
nothing at all; solution was to dissect the variable like
"$SomeTextSelect/self::text()[normalize-space(.) != string(.)]"]