pgsql: Enforce non-parallel plan when calling current_schema() innewly

Enforce non-parallel plan when calling current_schema() in newly-added test

current_schema() gets called in the recently-added regression test from
c5660e0, and can be used in a parallel context, causing its call to fail
when creating a temporary schema.

Per buildfarm members crake and lapwing.

Discussion: https://postgr.es/m/20190118005949.GD1883@paquier.xyz

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/08b53281f4444efb2d5bcc44d6293b05d1aa1335

Modified Files
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src/test/regress/expected/temp.out | 5 ++++-
src/test/regress/sql/temp.sql      | 5 ++++-
2 files changed, 8 insertions(+), 2 deletions(-)