feat: improvements to test_extern command (#3075)

Two improvements
[suggested](https://github.com/leanprover/lean4/pull/2970#issuecomment-1853436906)
by @digama0 after the initial PR was merged.

* Allow testing `implemented_by` attributes as well.
* Use `DecidableEq` rather than `BEq` for stricter testing.
4 files changed