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

src/test/regress/expected/create_function_c.out

@@ -1,5 +1,5 @@
 --
--- CREATE_FUNCTION_0
+-- CREATE_FUNCTION_C
 --
 -- This script used to create C functions for other scripts to use.
 -- But to get rid of the ordering dependencies that caused, such

src/test/regress/expected/create_function_sql.out

@@ -1,5 +1,5 @@
 --
--- CREATE FUNCTION
+-- CREATE_FUNCTION_SQL
 --
 -- Assorted tests using SQL-language functions
 --

src/test/regress/parallel_schedule

@@ -47,13 +47,13 @@ test: copy copyselect copydml insert insert_conflict
 # More groups of parallel tests
 # Note: many of the tests in later groups depend on create_index
 # ----------
-test: create_function_0 create_misc create_operator create_procedure create_table create_type
+test: create_function_c create_misc create_operator create_procedure create_table create_type
 test: create_index create_index_spgist create_view index_including index_including_gist
 
 # ----------
 # Another group of parallel tests
 # ----------
-test: create_aggregate create_function_3 create_cast constraints triggers select inherit typed_table vacuum drop_if_exists updatable_views roleattributes create_am hash_func errors infinite_recurse
+test: create_aggregate create_function_sql create_cast constraints triggers select inherit typed_table vacuum drop_if_exists updatable_views roleattributes create_am hash_func errors infinite_recurse
 
 # ----------
 # sanity_check does a vacuum, affecting the sort order of SELECT *

src/test/regress/sql/create_function_c.sql

@@ -1,5 +1,5 @@
 --
--- CREATE_FUNCTION_0
+-- CREATE_FUNCTION_C
 --
 -- This script used to create C functions for other scripts to use.
 -- But to get rid of the ordering dependencies that caused, such

src/test/regress/sql/create_function_sql.sql

@@ -1,5 +1,5 @@
 --
--- CREATE FUNCTION
+-- CREATE_FUNCTION_SQL
 --
 -- Assorted tests using SQL-language functions
 --