Make FSM test portable.

In b0eaa4c51b, we allow FSM to be created only after 4 pages.  One of the
tests check the FSM contents and to do that it populates many tuples in
the relation.  The FSM contents depend on the availability of freespace in
the page and it could vary because of the alignment of tuples.

This commit removes the dependency on FSM contents.

Author: Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1KADF6K1bagr0--mGv3dMcZ%3DH_Z-Qtvdfbp5PjaC6PJJA%40mail.gmail.com

contrib/pageinspect/expected/page.out

@@ -35,24 +35,11 @@ ERROR:  relation "xxx" does not exist
 SELECT octet_length(get_raw_page('test_rel_forks', 'xxx', 0));
 ERROR:  invalid fork name
 HINT:  Valid fork names are "main", "fsm", "vm", and "init".
-SELECT * FROM fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
- fsm_page_contents 
--------------------
- 0: 39            +
- 1: 39            +
- 3: 39            +
- 7: 39            +
- 15: 39           +
- 31: 39           +
- 63: 39           +
- 127: 39          +
- 255: 39          +
- 511: 39          +
- 1023: 39         +
- 2047: 39         +
- 4095: 39         +
- fp_next_slot: 0  +
- 
+EXPLAIN (costs off, analyze on, timing off, summary off) SELECT * FROM
+        fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
+                         QUERY PLAN                         
+------------------------------------------------------------
+ Function Scan on fsm_page_contents (actual rows=1 loops=1)
 (1 row)
 
 SELECT get_raw_page('test_rel_forks', 0) = get_raw_page('test_rel_forks', 'main', 0);

contrib/pageinspect/sql/page.sql

@@ -22,7 +22,8 @@ SELECT octet_length(get_raw_page('test_rel_forks', 'vm', 1)) AS vm_1;
 SELECT octet_length(get_raw_page('xxx', 'main', 0));
 SELECT octet_length(get_raw_page('test_rel_forks', 'xxx', 0));
 
-SELECT * FROM fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
+EXPLAIN (costs off, analyze on, timing off, summary off) SELECT * FROM
+        fsm_page_contents(get_raw_page('test_rel_forks', 'fsm', 0));
 
 SELECT get_raw_page('test_rel_forks', 0) = get_raw_page('test_rel_forks', 'main', 0);