Adjust XML test case to avoid unstable behavior.

Adjust XML test case to avoid unstable behavior.

Buildfarm member bowerbird is (inconsistently) showing different
results for this test case since we enabled ASLR for MSVC builds.
It's not very clear whether that's a bug in its version of libxml2
or the test case is relying on nominally-undefined behavior, ie the
ordering of results from XPath's node().  It seems quite unlikely
that it's *our* bug though, and what's more, using node() adds
nothing to the test coverage so far as our code is concerned.
So, tweak the test to not use node().

For the moment, only change HEAD because we've only seen the
problem there.  Perhaps a case will emerge for back-patching.

Discussion: https://postgr.es/m/2655387.1661695793@sss.pgh.pa.us