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/sql/create_function_c.sql

@@ -0,0 +1,35 @@
+--
+-- 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
+-- functions are now made either in test_setup.sql or in the specific
+-- test script that needs them.  All that remains here is error cases.
+
+-- directory path and dlsuffix are passed to us in environment variables
+\getenv libdir PG_LIBDIR
+\getenv dlsuffix PG_DLSUFFIX
+
+\set regresslib :libdir '/regress' :dlsuffix
+
+--
+-- Check LOAD command.  (The alternative of implicitly loading the library
+-- is checked in many other test scripts.)
+--
+LOAD :'regresslib';
+
+-- Things that shouldn't work:
+
+CREATE FUNCTION test1 (int) RETURNS int LANGUAGE C
+    AS 'nosuchfile';
+
+-- To produce stable regression test output, we have to filter the name
+-- of the regresslib file out of the error message in this test.
+\set VERBOSITY sqlstate
+CREATE FUNCTION test1 (int) RETURNS int LANGUAGE C
+    AS :'regresslib', 'nosuchsymbol';
+\set VERBOSITY default
+SELECT regexp_replace(:'LAST_ERROR_MESSAGE', 'file ".*"', 'file "..."');
+
+CREATE FUNCTION test1 (int) RETURNS int LANGUAGE internal
+    AS 'nosuch';