Update `Exercise anatomy` in `CONTRIBUTING.md`

[ryanplusplus]

• The tests now seem to be in test_{exercise}.c at the top level, with some test-framework/unity*.* files that seem fairly standard. This conflicts with the Exercise anatomy section of CONTRIBUTING.md (probably true of many tracks!).

Originally posted by @colinleach in #1006

This was messed up when the exercise structure changed when we transitioned from v1 to v2 (or v2 to v3... I can't remember).

[wolf99]

Could be a good first issue for someone... ? ☺️