Avoid unexpected slowdown in vacuum regression test.

I noticed the "vacuum" regression test taking really significantly longer
than it used to on a slow machine.  Investigation pointed the finger at
commit e415b469, which added creation of
an index using an extremely expensive index function.  That function was
evidently meant to be applied only twice ... but the test re-used an
existing test table, which up till a couple lines before that had had over
two thousand rows.  Depending on timing of the concurrent regression tests,
the intervening VACUUMs might have been unable to remove those
recently-dead rows, and then the index build would need to create index
entries for them too, leading to the wrap_do_analyze() function being
executed 2000+ times not twice.  Avoid this by using a different table
that is guaranteed to have only the intended two rows in it.

Back-patch to 9.0, like the commit that created the problem.

src/test/regress/expected/vacuum.out

@@ -65,9 +65,9 @@ CREATE FUNCTION do_analyze() RETURNS VOID VOLATILE LANGUAGE SQL
 	AS 'ANALYZE pg_am';
 CREATE FUNCTION wrap_do_analyze(c INT) RETURNS INT IMMUTABLE LANGUAGE SQL
 	AS 'SELECT $1 FROM do_analyze()';
-CREATE INDEX ON vactst(wrap_do_analyze(i));
-INSERT INTO vactst VALUES (1), (2);
-ANALYZE vactst;
+CREATE INDEX ON vaccluster(wrap_do_analyze(i));
+INSERT INTO vaccluster VALUES (1), (2);
+ANALYZE vaccluster;
 ERROR:  ANALYZE cannot be executed from VACUUM or ANALYZE
 CONTEXT:  SQL function "do_analyze" statement 1
 SQL function "wrap_do_analyze" statement 1
@@ -75,9 +75,9 @@ VACUUM FULL pg_am;
 VACUUM FULL pg_class;
 VACUUM FULL pg_database;
 VACUUM FULL vaccluster;
-VACUUM FULL vactst;
 ERROR:  ANALYZE cannot be executed from VACUUM or ANALYZE
 CONTEXT:  SQL function "do_analyze" statement 1
 SQL function "wrap_do_analyze" statement 1
+VACUUM FULL vactst;
 DROP TABLE vaccluster;
 DROP TABLE vactst;

src/test/regress/sql/vacuum.sql

@@ -50,9 +50,9 @@ CREATE FUNCTION do_analyze() RETURNS VOID VOLATILE LANGUAGE SQL
 	AS 'ANALYZE pg_am';
 CREATE FUNCTION wrap_do_analyze(c INT) RETURNS INT IMMUTABLE LANGUAGE SQL
 	AS 'SELECT $1 FROM do_analyze()';
-CREATE INDEX ON vactst(wrap_do_analyze(i));
-INSERT INTO vactst VALUES (1), (2);
-ANALYZE vactst;
+CREATE INDEX ON vaccluster(wrap_do_analyze(i));
+INSERT INTO vaccluster VALUES (1), (2);
+ANALYZE vaccluster;
 
 VACUUM FULL pg_am;
 VACUUM FULL pg_class;