Commit 31a89185 added some tests in
plpgsql.sql that used a function rather unthinkingly named "foo()".
However, rangefuncs.sql has some much older tests that create a function
of that name, and since these test scripts run in parallel, there is a
chance of failures if the timing is just right.  Use another name to
avoid that.  Per buildfarm (failure seen today on "hamerkop", but
probably it's happened before and not been noticed).