pgsql: Make some subquery-using test cases a bit more robust.

Make some subquery-using test cases a bit more robust.

These test cases could be adversely affected by an upcoming change
to allow pullup of FROM-less subqueries. Tweak them to ensure that
they'll continue to test what they did before.

Discussion: https://postgr.es/m/5395.1539275668@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b403ea43e40aae3ebb03cf3844cf84b43ab34b8a

Modified Files
--------------
src/test/regress/expected/select.out | 6 +++---
src/test/regress/expected/select_parallel.out | 15 +++++++--------
src/test/regress/sql/select.sql | 6 +++---
src/test/regress/sql/select_parallel.sql | 4 ++--
4 files changed, 15 insertions(+), 16 deletions(-)