pgsql: Provide deterministic order for catalog queries in partition_spl

Provide deterministic order for catalog queries in partition_split.sql

System catalog tables are subject to modification by parallel tests.  This
is the source of instability when querying them without explicit ORDER BY.
This commit adds explicit ORDER BY to system catalog queries in
partition_split.sql to stabilize the result.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/695264.1716578979%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d53a4286d772c50ad7a8ff72ca637de613532592

Modified Files
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src/test/regress/expected/partition_split.out | 34 +++++++++++++--------------
src/test/regress/sql/partition_split.sql      | 34 +++++++++++++--------------
2 files changed, 34 insertions(+), 34 deletions(-)