tests: Turn tested hooks into fixtures.

This is in preparation for passing configuration to hooks.
That'll be a big change already, so I wanted this to be a
boring diff first.

tests: Add check_post_meta.

This makes it simple to conveniently check all posting metadata in tests.

plugin: Transform posting hooks into transaction hooks.

I feel like posting hooks a case of premature optimization in early
development. This approach reduces the number of special cases in
the code and allows us to more strongly reason about hooks in the
type system.