irmin/test: tighten test module conventions (E600/E615)
- Rename test_helpers.ml{,i} to helpers.ml{,i}: it holds test helpers,
not an Alcotest suite, so the test_ prefix mis-signals. Updates
callers in test_atproto{,_ext,_tree}, test_pds_interop, test_stores.
- Add test_*.mli files for test_admin, test_atproto{,_ext,_tree},
test_gzip so every test module advertises only `suite`.
- Collapse test_stores `suites` list to a single `suite` (one generic
store runner per backend) to match the one-suite-per-module convention.
- Wire the previously-missing test suites into test.ml so they actually
run.