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/sql/plpython_void.sql

@@ -0,0 +1,22 @@
+--
+-- 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";
+SELECT test_void_func2(); -- should fail
+SELECT test_return_none(), test_return_none() IS NULL AS "is null";