test_ddl_deparse: Don't use pg_class as source for a matview

Doing so causes a pg_upgrade of a database containing these objects to
fail whenever pg_class changes.  And it's pointless anyway: we have more
interesting tables anyhow.

Discussion: https://postgr.es/m/CAD5tBc+s8pW9WvH2+_z=B4x95FD4QuzZKcaMpff_9H4rS0VU1A@mail.gmail.com

src/test/modules/test_ddl_deparse/expected/matviews.out

@@ -2,7 +2,7 @@
 -- Materialized views
 --
 CREATE MATERIALIZED VIEW pg_class_mv AS
-  SELECT * FROM pg_class LIMIT 1 WITH NO DATA;
+  SELECT * FROM datatype_table LIMIT 1 WITH NO DATA;
 NOTICE:  DDL test: type simple, tag CREATE MATERIALIZED VIEW
 REFRESH MATERIALIZED VIEW pg_class_mv;
 NOTICE:  DDL test: type simple, tag REFRESH MATERIALIZED VIEW

src/test/modules/test_ddl_deparse/sql/matviews.sql

@@ -3,6 +3,6 @@
 --
 
 CREATE MATERIALIZED VIEW pg_class_mv AS
-  SELECT * FROM pg_class LIMIT 1 WITH NO DATA;
+  SELECT * FROM datatype_table LIMIT 1 WITH NO DATA;
 
 REFRESH MATERIALIZED VIEW pg_class_mv;