pgsql: Fix test case from b0c5b215d.

Fix test case from b0c5b215d.

I'd not checked that this iteration of the test actually worked
with a bootstrap superuser not named 'postgres'.  It didn't,
because the coercion rules for CASE caused us to try to cast
the 'postgres' literal to regrole.  Mea culpa.

Per buildfarm (via Alexander Korotkov)

Discussion: https://postgr.es/m/CAPpHfdsV=iTvH6B858hnH1bLgewYH6cdTnO_eOOw9EOa8kehkA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9d9ece4c16dbbaf3b9d60c2fe201b8e99a407be3

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