chore: turn batteries tests into a library

[edegeltje]

see also this PR to mathlib, this discussion of that PR on zulip, and the discussion leading to this PR

tldr; this allows you to write tests for importing behaviour without "contaminating" the library proper with test data.

[leanprover-community-bot]

Mathlib CI status (docs):

• ✅ Mathlib branch batteries-pr-testing-1024 has successfully built against this PR. (2024-11-04 17:23:40) View Log