pgsql: Rename create_function_N test scripts for clarity.

Rename create_function_N test scripts for clarity.

Rename create_function_0 to create_function_c, and create_function_3
to create_function_sql, to establish their charters more clearly.
This should also reduce confusion versus our underscore-digit
convention for naming variant expected-files.

I separated this from the previous commit on the premise that keeping
the renaming distinct might make "git blame" tracking easier.

Discussion: https://postgr.es/m/1114748.1640383217@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2da896182ce11240774af6c4d769777f90a09536

Modified Files
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.../regress/expected/{create_function_0.out => create_function_c.out} | 2 +-
.../expected/{create_function_3.out => create_function_sql.out}       | 2 +-
src/test/regress/parallel_schedule                                    | 4 ++--
src/test/regress/sql/{create_function_0.sql => create_function_c.sql} | 2 +-
.../regress/sql/{create_function_3.sql => create_function_sql.sql}    | 2 +-
5 files changed, 6 insertions(+), 6 deletions(-)