pgsql: Harden new parallel string_agg/array_agg regression test

Harden new parallel string_agg/array_agg regression test

Per buildfarm member mandrill, it seems that
max_parallel_workers_per_gather may not always be set to the default value
of 2 when the new test added in 16fd03e95 is executed.  Here let's just
explicitly set that to 2 so that the planner never opts to use more than
that many parallel workers.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/67c5b8840fcad07eeecd9af517b9b1ff09a3cf8e

Modified Files
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src/test/regress/expected/aggregates.out | 1 +
src/test/regress/sql/aggregates.sql      | 1 +
2 files changed, 2 insertions(+)