Move TimeHelperTest to TimeTravelTest from `as/test_test.rb`

/play whoops

On Tuesday, August 19, 2014, Rafael Mendonça França <
notifications@github.com> wrote:

> In activesupport/test/time_travel_test.rb:
>
> > @@ -0,0 +1,68 @@
> > +class TimeTravelTest < ActiveSupport::TestCase
>
> No need to require abstract_unit?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/rails/rails/pull/16564/files#r16434801>.
>