pgsql: Extend SQL function tests lightly

Extend SQL function tests lightly

The basic tests that defined SQL functions didn't actually run the
functions to see if they worked.  Add that, and also fix a minor
mistake in a function that was revealed by this.  (This is not a
question of test coverage, since there are other places where SQL
functions are run, but it is a bit of a silly test design.)

Discussion: https://www.postgresql.org/message-id/flat/1c11f1eb-f00c-43b7-799d-2d44132c02d7@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/11b80d900fe4297e8e4bc231f6a41b53d604ed9e

Modified Files
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src/test/regress/expected/create_function_3.out | 20 +++++++++++++++++++-
src/test/regress/sql/create_function_3.sql      |  6 +++++-
2 files changed, 24 insertions(+), 2 deletions(-)