Fix bitrot and enable db_size_functions test.

I added these tests in commit f8a80aeb, but forgot to add the tests
to the test schedule. Add it now.

The OID of template0 database isn't stable across versions, so change the
test to use template1 instead, which has hard-coded OID 1.

src/test/regress/expected/db_size_functions.out

@@ -16,7 +16,7 @@ select pg_database_size('template0'::name) between 40000000 and 200000000;
  t
 (1 row)
 
-select pg_database_size(12510::oid) = pg_database_size('template0'::name);
+select pg_database_size(1::oid) = pg_database_size('template1'::name);
  ?column? 
 ----------
  t

src/test/regress/greenplum_schedule

@@ -101,6 +101,8 @@ test: sreh
 # vacuum from removing dead tuples.
 test: gp_toolkit
 
+test: db_size_functions
+
 # FIXME: These tests no longer work, because they try to set
 # gp_interconnect_type, which doesn't work:
 # ERROR:  parameter "gp_interconnect_type" cannot be set after connection start

src/test/regress/sql/db_size_functions.sql

@@ -13,7 +13,7 @@
 -- As of this writing, the total size of template0 database, across three segments,
 -- is 67307536 bytes.
 select pg_database_size('template0'::name) between 40000000 and 200000000;
-select pg_database_size(12510::oid) = pg_database_size('template0'::name);
+select pg_database_size(1::oid) = pg_database_size('template1'::name);
 
 -- 19713632 bytes, as of this writing
 select pg_tablespace_size('pg_global'::name) between 10000000 and 50000000;