Some relations with LZ4 used as the toast compression methods have been
left around in the main regression test suite to stress pg_upgrade, but
pg_dump, that includes tests much more picky in terms of output
generated, had no actual coverage with this feature.

Similarly to collations, tests only working with LZ4 are tracked with an
additional flag, and this uses TestLib::check_pg_config() to check if
the build supports LZ4 or not.  This stresses more scenarios with
tables, materialized views and pg_dump --no-toast-compression.

Author: Dilip Kumar
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAFiTN-twgPmohG7qj1HXhySq16h123y5OowsQR+5h1YeZE9fmQ@mail.gmail.com