pgsql: Tweak tests related to oid8 in amcheck and test_decoding

Tweak tests related to oid8 in amcheck and test_decoding

Some of the tests introduced by c68cba09dd7f in these two modules want
to rely on out-of-line values (compressed and not compressed), but the
values inserted that should be compressed were not (STORAGE EXTENDED).

The tests are updated to do what they should, reusing a cheap trick to
ensure out-of-line values coming from a different test in test_decoding.
Some checks are added to ensure that the expected set of values is
inserted.

Reported-by: Rui Zhao <zhaorui126(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHWVJhGxVc4CpDd+mb=6YbeL2KHDUtsUC=R68y7NeMg1cO8QMg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/87c6ef14f11705e690a9c54f91e0d97977ce333f

Modified Files
--------------
contrib/amcheck/expected/check_heap.out | 33 ++++++++++++++++++++++++--------
contrib/amcheck/sql/check_heap.sql | 21 ++++++++++++--------
contrib/test_decoding/expected/toast.out | 20 +++++++++++++++----
contrib/test_decoding/sql/toast.sql | 10 ++++++++--
4 files changed, 62 insertions(+), 22 deletions(-)