Drop unlogged table after test is done

Another test is constructed on top of regression tests, which does not
work correctly with unlogged tables.  For now, cope with that by making
sure no unlogged table is left behind.

Per buildfarm pink after 4fb5c794e5.

src/test/regress/expected/spgist.out

@@ -41,6 +41,7 @@ select count(*)
      3
 (1 row)
 
+drop table spgist_box_tbl;
 -- The point opclass's choose method only uses the spgMatchNode action,
 -- so the other actions are not tested by the above. Create an index using
 -- text opclass, which uses the others actions.

src/test/regress/sql/spgist.sql

@@ -45,6 +45,7 @@ create index spgist_box_idx on spgist_box_tbl using spgist (b);
 select count(*)
   from (values (point(5,5)),(point(8,8)),(point(12,12))) v(p)
  where exists(select * from spgist_box_tbl b where b.b && box(v.p,v.p));
+drop table spgist_box_tbl;
 
 -- The point opclass's choose method only uses the spgMatchNode action,
 -- so the other actions are not tested by the above. Create an index using