Split the plpython regression test into test cases arranged by topic, instead

of the previous monolithic setup-create-run sequence, that was apparently
inherited from a previous test infrastructure, but makes working with the
tests and adding new ones weird.

src/pl/plpython/expected/plpython_void.out

@@ -0,0 +1,29 @@
+--
+-- Tests for functions that return void
+--
+CREATE FUNCTION test_void_func1() RETURNS void AS $$
+x = 10
+$$ LANGUAGE plpythonu;
+-- illegal: can't return non-None value in void-returning func
+CREATE FUNCTION test_void_func2() RETURNS void AS $$
+return 10
+$$ LANGUAGE plpythonu;
+CREATE FUNCTION test_return_none() RETURNS int AS $$
+None
+$$ LANGUAGE plpythonu;
+-- Tests for functions returning void
+SELECT test_void_func1(), test_void_func1() IS NULL AS "is null";
+ test_void_func1 | is null 
+-----------------+---------
+                 | f
+(1 row)
+
+SELECT test_void_func2(); -- should fail
+ERROR:  PL/Python function with return type "void" did not return None
+CONTEXT:  PL/Python function "test_void_func2"
+SELECT test_return_none(), test_return_none() IS NULL AS "is null";
+ test_return_none | is null 
+------------------+---------
+                  | t
+(1 row)
+