Most xmlpatterns tests use private QtCore and QtXmlPatterns headers,
so `QT += core-private xmlpatterns-private' is required.

In Qt5, $$QT_SOURCE_TREE refers to the source tree of `qtbase'.
Change it to `$$QT.xmlpatterns.sources'.

Reviewed-by: Jason McDonald
Change-Id: I711585819d2ce5a8b151e76ed79fa13be1f29379