pgsql: Add battery of tests related TOAST tables with oid8

Add battery of tests related TOAST tables with oid8

The tests added by this commit cover a large ground in terms of the
introduction of oid8 in TOAST tables, with the following areas covered,
with all the gaps found during the development of the feature:
- Tables with oid8 TOAST tables in general, under strings.sql.
- ALTER TABLE with attribute manipulations in transactions and different
toast_table_value settings.
- Relation rewrites, not affecting the TOAST relation after initial
creation with VACUUM FULL, CLUSTER, REPACK.
- pg_column_compression().
- test_decoding, for external and extended TOAST data.
- amcheck, external and extended storage with heap checked.
- UPDATE with out-of-line datums that belongs to another TOAST table.
This offers coverage for toast_tuple_init(), where external vartags
could be mixed once multiple on-disk external TOAST pointers can
co-exist. This pattern can be triggered with asan/ubsan, with an
out-of-bound memcpy when unpatched.

These tests are added before the new vartag_external, mostly as a matter
of avoiding dead code in the tree.

Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Greg Burd <greg(at)burd(dot)me>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>
Discussion: https://postgr.es/m/af19kUjwjhaoUTLn@paquier.xy

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c68cba09dd7fb2cca1fdddfcf8e0fe206e97a8a4

Modified Files
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contrib/amcheck/expected/check_heap.out | 40 +++
contrib/amcheck/sql/check_heap.sql | 27 ++
contrib/test_decoding/expected/toast.out | 39 +++
contrib/test_decoding/sql/toast.sql | 20 ++
src/test/regress/expected/compression.out | 41 ++++
src/test/regress/expected/strings.out | 394 +++++++++++++++++++++++++++---
src/test/regress/sql/compression.sql | 22 ++
src/test/regress/sql/strings.sql | 241 ++++++++++++++----
8 files changed, 746 insertions(+), 78 deletions(-)