Fix node kind checks

[GuntherRademacher]

The changes in bbd7744 have caused these differences in the behaviour of location steps:

• descendant-or-self steps can lose their name test, when merged with a subsequent step. This makes this query return an empty sequence:

  declare function local:f($html as node()) {
    $html//text() except $html/descendant-or-self::b//node()
  };
  let $html := <a>X</a>
  return local:f($html)

• a narrower type of a name test in a self step may be overridden by the broader node(), e.g. in

  declare function local:f($html as node()) {
    $html/self::b
  };
  local:f#1

This is caused by checks in Path.mergeStep and Step.assignType that were changed in bbd7744. With the fix here, both checks now require the test to be one of the NodeTest wildcard singletons (NodeTest.NODE / JNODE / XNODE), rather than to just carry the same kind.