tests: More XPath tests

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1020 99668b35-9821-0410-8761-19e4c4f06640
XPath: Optimize //name queries when possible

//name means /descendant-or-self::node()/child::name, but we frequently
can replace it with /descendant::name. This means we do not have to build
up a temporary node set with all descendants that can lead to 3x speedups.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1021 99668b35-9821-0410-8761-19e4c4f06640
tests: Add one more XPath optimization test

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1022 99668b35-9821-0410-8761-19e4c4f06640