pgsql: Add a test for creating an index on a whole-row expression.

Add a test for creating an index on a whole-row expression.

Surprisingly, we have no existing test for this. Had this test
been present before commit 570e2fcc041a55ba8991a640cc3f3f0e122feac3
the Assert added in commit c98ad086ad9b1ca9dbb2725f246298fa8450d82f
would have caught the bug.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CA+TgmoacixUZVvi00hOjk_d9B4iYKswWP1gNqQ8Vfray-AcOCA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4647ee2da37c106f0b964171227df956082d39a4

Modified Files
--------------
src/test/regress/expected/indexing.out | 11 +++++++++++
src/test/regress/sql/indexing.sql      | 12 ++++++++++++
2 files changed, 23 insertions(+)