SI-8253 Fix incorrect parsing of <elem xmlns={f("a")}/>

The spliced application was placed in the `attrMap` in
`SymbolicXMLBuilder` and later incorrectly matched by a pattern
intended only to match:

    xml.Text(s)

That attribute value is generated by parsing:

   <elem xmlns='a'/>

So the net effect was that the two fragments of XML were identical!

This commit sharpens up the match to really look for a syntactic
`_root_.scala.xml.Text("...")`.

The test just prints the parse trees of a variety of cases, as we
we should not test the modularized XML library in scala/scala.

Review by @som-snytt