pgsql: Further improve jsonb_sqljson parallel test

Further improve jsonb_sqljson parallel test

Instead of using a very large table, use some settings to encourage use
of parallelism. Also, drop the table so it doesn't upset the recovery
test.

per suggestion from Andres Freund

Discussion: https://postgr.es/m/20220406022118.3ocqvhxr6kciw5am@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/14d3f24fa8a21f8a7e66f1fc60253a1e11410bf3

Modified Files
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src/test/regress/expected/jsonb_sqljson.out | 23 +++++++++++++++--------
src/test/regress/sql/jsonb_sqljson.sql      | 14 +++++++++++++-
2 files changed, 28 insertions(+), 9 deletions(-)