Add option to set parallel test worker count to the physical core count of the machine (#34735)

* Add option to set parallel test worker count to the physical core count of the machine

Also, use the physical core count of the machine as
the default number of workers, and  generate the `test_helper.rb` file
with `parallelize(workers: :number_of_processors)`

Closes #34734

* Ensure that we always test parallel testing

Since #34734 we decided to use the physical core count of the machine as
the default number of workers in the parallel testing, we need to
ensure that some tests use at least 2 workers because we could
run those tests on VM that has only 1 physical core.
It also fixes tests failures on the CI since Travis server we are using
has only one physical core.
See https://travis-ci.org/rails/rails/jobs/469281088#L2352

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