pgsql: Forward-patch regression test for "could not find pathkey item t

Forward-patch regression test for "could not find pathkey item to sort".

Commit a87c729153e372f3731689a7be007bc2b53f1410 already fixed the bug this
is checking for, but the regression test case it added didn't cover this
scenario.  Since we managed to miss the fact that there was a bug at all,
it seems like a good idea to propagate the extra test case forward to HEAD.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/344eed91e9d5bfea698b30351abde69ea4e893a8

Modified Files
--------------
src/test/regress/expected/union.out |   22 ++++++++++++++++++++++
src/test/regress/sql/union.sql      |   16 ++++++++++++++++
2 files changed, 38 insertions(+)