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-
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