test(file_store): `last_write_is_short`

test(file_store): `last_write_is_short`

This test simulates a situation where the last write to the db is short.

Aggregating the changeset after reopening the file should return an
error (which includes a partially-aggregated changeset) containing an
aggregation of changesets that were fully written.

At this point, the test re-writes the final changeset (and this time it
successfully writes in full).

The file should be recoverable with all changesets, including the last
one.