pgsql: Try to stabilize flappy test result.

Try to stabilize flappy test result.

This recently-added test case checks the plan of an inner join
between two identical tables. It's just chance which join order
the planner will pick, and in the presence of any variation in
the underlying statistics, the displayed plan might change.
Add a WHERE condition to break the cost symmetry and hopefully
stabilize matters.

(We're still trying to understand exactly why the underlying
statistics aren't as stable as intended, but this seems like
a good change anyway, since this test would surely bite us
again in future.)

While here, clean up assorted comment spelling, grammar, and
whitespace problems.

Discussion: https://postgr.es/m/4168116.1711720146@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c2df2ed90a82fc0acd07ed88ac433350edfd6b54

Modified Files
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src/test/regress/expected/subselect.out | 14 ++++++++------
src/test/regress/sql/subselect.sql | 13 +++++++------
2 files changed, 15 insertions(+), 12 deletions(-)