pgsql: Adjust regression test cases added by commit ab7271677.

Adjust regression test cases added by commit ab7271677.

I suppose it is a copy-and-paste error that this test doesn't actually
test the "Parallel Append with both partial and non-partial subplans"
case (EXPLAIN alone surely doesn't qualify as a test of executor
behavior). Fix that.

Also, add cosmetic aliases to make it possible to tell apart these
otherwise-identical test cases in log_statement output.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/979a36c3894db0a4b0d6b4b20fc861a0bbe3271c

Modified Files
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src/test/regress/expected/select_parallel.out | 12 +++++++++---
src/test/regress/sql/select_parallel.sql | 7 ++++---
2 files changed, 13 insertions(+), 6 deletions(-)