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