`Pattern#from_sting` was introduced in bb207ea7 to support creating
patterns from strings in tests (removing a conditional in the initialize
method that had been there only for the sake of those tests).

`Pattern#build` was introduced in 947ebe9a, and was similarly used only
in tests.

My understanding is that [`Mapping#path`][path] is the only place where
we initialize a `Path::Pattern` in library code.

Since these two methods appear to be used only in tests, this
commit moves them out of the class and into a test helper.

My reasoning for doing this is that I am doing some performance work
that may involve a modification to how `Path::Pattern`s get initialized,
and I will have more confidence in that work if I know for sure that
these methods are test helpers that can be modified freely.

[path]: https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/routing/mapper.rb#L192-L194