Improve test coverage of ginvacuum.c.

Add a test case that exercises vacuum's deletion of empty GIN
posting pages.  Since this is a temp table, it should now work
reliably to delete a bunch of rows and immediately VACUUM.
Before the preceding commit, this would not have had the desired
effect, at least not in parallel regression tests.

Discussion: https://postgr.es/m/3490536.1598629609@sss.pgh.pa.us

src/test/regress/expected/gin.out

@@ -264,6 +264,27 @@ select count(*) from t_gin_test_tbl where j @> '{}'::int[];
  20006
 (1 row)
 
+-- test vacuuming of posting trees
+delete from t_gin_test_tbl where j @> array[2];
+vacuum t_gin_test_tbl;
+select count(*) from t_gin_test_tbl where j @> array[50];
+ count 
+-------
+     0
+(1 row)
+
+select count(*) from t_gin_test_tbl where j @> array[2];
+ count 
+-------
+     0
+(1 row)
+
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+ count 
+-------
+     6
+(1 row)
+
 reset enable_seqscan;
 reset enable_bitmapscan;
 drop table t_gin_test_tbl;

src/test/regress/sql/gin.sql

@@ -159,6 +159,14 @@ explain (costs off)
 select count(*) from t_gin_test_tbl where j @> '{}'::int[];
 select count(*) from t_gin_test_tbl where j @> '{}'::int[];
 
+-- test vacuuming of posting trees
+delete from t_gin_test_tbl where j @> array[2];
+vacuum t_gin_test_tbl;
+
+select count(*) from t_gin_test_tbl where j @> array[50];
+select count(*) from t_gin_test_tbl where j @> array[2];
+select count(*) from t_gin_test_tbl where j @> '{}'::int[];
+
 reset enable_seqscan;
 reset enable_bitmapscan;