[CI SKIP] Corrections to testing guide.

Partially revert https://github.com/rails/rails/commit/c31e638a37946a120f6dbf207127d36beea5ab85.
There was no good reason to remove `You can also simply run `rake` as Rails will run all the tests by default` as running `rake` will still allow users to run all the tests. 

The above is also not correct as there isn't a default when you run `rake test`. (i.e the behavior of `rake test` will always be the same.)