More extensive `blockio` tests (for `FreeBSD` support)

For `FreeBSD` support, we are currently only testing `blockio` in CI (not `lsm-tree`) because that is where the `FreeBSD`-specific code lives. However, the `blockio` tests are not very extensive and so we are not exercising all parts of the `FreeBSD` backend. We should write more tests so that we cover the whole of the `HasBlockIO` API at least. Other (future) backends will also benefit from this.

Note: the core Linux, MacOS, and Windows `blockio` backends are already extensively tested indirectly through testing `lsm-tree`, so they benefit a little less from more extensive tests, but it's still very useful.

Start by locating the existing blockio tests and the HasBlockIO API, then compare their coverage with the FreeBSD-specific backend used in CI. Extend the tests to exercise the full HasBlockIO API and verify the blockio test suite passes for FreeBSD; lsm-tree coverage is useful context but is not the primary target.

Written by the indexing model from the issue text.