pgsql: Disable force_parallel_mode in libpq_pipeline

Disable force_parallel_mode in libpq_pipeline

Some buildfarm animals with force_parallel_mode=regress were failing
this test because the error is reported in a parallel worker quicker
than the rows that succeed.

Take the opportunity to move the SET of lc_messages out of the traced
section, because it's not very interesting.

Diagnosed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/3304521.1617221724@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a6d3dea8e5e0c8a0df2f95d66b6c3903a4354ca0

Modified Files
--------------
src/test/modules/libpq_pipeline/libpq_pipeline.c | 12 ++++++++----
.../libpq_pipeline/traces/disallowed_in_pipeline.trace | 3 ---
src/test/modules/libpq_pipeline/traces/multi_pipelines.trace | 3 ---
src/test/modules/libpq_pipeline/traces/pipeline_abort.trace | 3 ---
src/test/modules/libpq_pipeline/traces/prepared.trace | 3 ---
src/test/modules/libpq_pipeline/traces/simple_pipeline.trace | 3 ---
src/test/modules/libpq_pipeline/traces/singlerow.trace | 3 ---
src/test/modules/libpq_pipeline/traces/transaction.trace | 3 ---
8 files changed, 8 insertions(+), 25 deletions(-)