Restructure base85 unit tests to support inlining and multiple encoding pathways

Summary:
Ref T13130. I want to take a crack at improving performance here, but two possible approaches (inlining the actual encoding; using integers if they're big enough) aren't easy to test right now.

Restructure the tests so they can support these kinds of refactoring.

The "32bit" and "64bit" modes currently do the same thing, but I expect to introduce introduce separate encoding pathways in a future change, if the profiler says it actually helps.

(I'll hold this and everything that comes after it until I make meaningful performance improvements.)

Test Plan: Ran arc unit, got passes on tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19407