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@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3304521.1617221724@sss.pgh.pa.us

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -438,6 +438,7 @@ test_pipeline_abort(PGconn *conn)
 	if (PQresultStatus(res) != PGRES_PIPELINE_SYNC)
 		pg_fatal("Unexpected result code %s from pipeline sync",
 				 PQresStatus(PQresultStatus(res)));
+	fprintf(stderr, "ok\n");
 
 	/* Test single-row mode with an error partways */
 	if (PQsendQuery(conn, "SELECT 1.0/g FROM generate_series(3, -1, -1) g") != 1)
@@ -1308,6 +1309,13 @@ main(int argc, char **argv)
 		exit_nicely(conn);
 	}
 
+	res = PQexec(conn, "SET lc_messages TO \"C\"");
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
+		pg_fatal("failed to set lc_messages: %s", PQerrorMessage(conn));
+	res = PQexec(conn, "SET force_parallel_mode = off");
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
+		pg_fatal("failed to set force_parallel_mode: %s", PQerrorMessage(conn));
+
 	/* Set the trace file, if requested */
 	if (tracefile != NULL)
 	{
@@ -1320,10 +1328,6 @@ main(int argc, char **argv)
 						PQTRACE_SUPPRESS_TIMESTAMPS | PQTRACE_REGRESS_MODE);
 	}
 
-	res = PQexec(conn, "SET lc_messages TO \"C\"");
-	if (PQresultStatus(res) != PGRES_COMMAND_OK)
-		pg_fatal("failed to set lc_messages: %s", PQerrorMessage(conn));
-
 	if (strcmp(testname, "disallowed_in_pipeline") == 0)
 		test_disallowed_in_pipeline(conn);
 	else if (strcmp(testname, "multi_pipelines") == 0)

src/test/modules/libpq_pipeline/traces/disallowed_in_pipeline.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	13	Query	 "SELECT 1"
 B	33	RowDescription	 1 "?column?" NNNN 0 NNNN 4 -1 0
 B	11	DataRow	 1 1 '1'

src/test/modules/libpq_pipeline/traces/multi_pipelines.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	21	Parse	 "" "SELECT $1" 1 NNNN
 F	19	Bind	 "" "" 0 1 1 '1' 1 0
 F	6	Describe	 P ""

src/test/modules/libpq_pipeline/traces/pipeline_abort.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	42	Query	 "DROP TABLE IF EXISTS pq_pipeline_demo"
 B	123	NoticeResponse	 S "NOTICE" V "NOTICE" C "00000" M "table "pq_pipeline_demo" does not exist, skipping" F "SSSS" L "SSSS" R "SSSS" \x00
 B	15	CommandComplete	 "DROP TABLE"

src/test/modules/libpq_pipeline/traces/prepared.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	68	Parse	 "select_one" "SELECT $1, '42', $1::numeric, interval '1 sec'" 1 NNNN
 F	16	Describe	 S "select_one"
 F	4	Sync

src/test/modules/libpq_pipeline/traces/simple_pipeline.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	21	Parse	 "" "SELECT $1" 1 NNNN
 F	19	Bind	 "" "" 0 1 1 '1' 1 0
 F	6	Describe	 P ""

src/test/modules/libpq_pipeline/traces/singlerow.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	38	Parse	 "" "SELECT generate_series(42, $1)" 0
 F	20	Bind	 "" "" 0 1 2 '44' 1 0
 F	6	Describe	 P ""

src/test/modules/libpq_pipeline/traces/transaction.trace

-F	27	Query	 "SET lc_messages TO "C""
-B	8	CommandComplete	 "SET"
-B	5	ReadyForQuery	 I
 F	79	Query	 "DROP TABLE IF EXISTS pq_pipeline_tst;CREATE TABLE pq_pipeline_tst (id int)"
 B	122	NoticeResponse	 S "NOTICE" V "NOTICE" C "00000" M "table "pq_pipeline_tst" does not exist, skipping" F "SSSS" L "SSSS" R "SSSS" \x00
 B	15	CommandComplete	 "DROP TABLE"