Make test_helper both accessible to main.rs and lib.rs

From this idea.

This:

Gets rid of Stub::SIMPLE_REFERENCE.
Exports as pub test_helper but should be obvious that it's not intended for public use.
Allows for easier synchronization between CG and the reference test (Ideally a quick script to update the json should be located on fixtures/puzzles?).

NOTE: In the future, the reference fixture, and the fixture used now for solution.rs should ideally be separated.

Maybe fixture_loader is a more suggestive name than test_helper.

Andriamanitra / coctus