irmin-pack: improve test suite coverage of Conf values
irmin-pack: improve test suite coverage of Conf values

Changes the `irmin-pack` test suite instantiation to test multiple
choices for the following configuration options:

- ``indexing_strategy = always | minimal`` (as before)
- ``entries = 2 | 32``
- ``stable_hash = 3 | 256``
- ``contents_length_header = None | Some `Varint``
- ``inode_child_order = `Hash_bits | `Seeded``

This should provide better coverage overall.

mirage/irmin,mirage/irmin