pgsql: Move regression test listing of builtin leakproof functions to o

Move regression test listing of builtin leakproof functions to opr_sanity.sql.

The original location in create_function_3.sql didn't invite the close
structinity warranted for adding new leakproof functions. Add comments
to the test explaining that functions should only be added after
careful consideration and understanding what a leakproof function is.

Per complaint from Tom Lane after 5eebb8d954ad.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e0cb4aa89dd61cb1a76d348ced634681255b6a07

Modified Files
--------------
src/test/regress/expected/create_function_3.out |  238 ----------------------
src/test/regress/expected/opr_sanity.out        |  249 ++++++++++++++++++++++-
src/test/regress/sql/create_function_3.sql      |   14 --
src/test/regress/sql/opr_sanity.sql             |   25 ++-
4 files changed, 268 insertions(+), 258 deletions(-)