Pass ConnectionPool to tests

[parsonsmatt]

This PR inverts control in the test suite. Instead of having each test suite provide a run function which runs the database action, the test suite expects a ConnectionPool to be made available to it. Then the various backends use beforeAll :: IO ConnectionPool -> SpecWith ConnectionPool -> Spec to make it happen.

A further advantage is that we'll be able to use beforeWith :: (ConnectionPool -> IO ConnectionPool) -> SpecWith ConnectionPool -> SpecWith ConnectionPool to implement things like, y'know, inserting test/setup data into the database, and afterWith to clean it up.

Before submitting your PR, check that you've:

• [ ] Bumped the version number.
• [ ] Documented new APIs with Haddock markup.
• [ ] Added @since declarations to the Haddock.
• [ ] Ran stylish-haskell and otherwise adhered to the style guide.

After submitting your PR:

• [ ] Update the Changelog.md file with a link to your PR.
• [ ] Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts).