Extract out with_env_tz helper method.

It’s used at so many places that extracting it out into a helper file
is worth doing.

cc @senny
I was actually following the structure of `multibyte_test_helpers.rb` and `dependencies_test_helpers.rb`, both in `test` folder, with plural `Helpers`. :stuck_out_tongue: Should I make the change or keep it as it is?