pgsql: Fix unstable test in select_parallel.sql

Fix unstable test in select_parallel.sql

One test case added in 22d946b0f verifies the plan of a non-parallel
nestloop join. The planner's choice of join order is arbitrary, and
slight variations in underlying statistics could result in a different
displayed plan. To stabilize the test result, here we enforce the
join order using a lateral join.

While here, modify the test case to verify that parallel nestloop join
is not generated if the inner path is not parallel-safe, which is what
we wanted to test in 22d946b0f.

Reported-by: Alexander Lakhin as per buildfarm
Author: Richard Guo
Discussion: https://postgr.es/m/7c09a439-e48d-5460-cfa0-a371b1a57066@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7e187a7386cc922c8f770c0460bfc43f4806bd15

Modified Files
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src/test/regress/expected/select_parallel.out | 17 +++++++++++------
src/test/regress/sql/select_parallel.sql | 11 ++++++++---
2 files changed, 19 insertions(+), 9 deletions(-)