Refactor split `Graphics/` tests into independent testing files.

[adamkewley]

They are currently all in TestRenderer because that's where they organically started and kept growing.

The graphics API now has separate classes (e.g. Material), so it makes sense that the test suites should match 1:1 (e.g. TestMaterial)

[adamkewley]

Dropping from 0.5.2 (past due)