Consolidate Tests

[parsonsmatt]

This PR consolidates the test suite and provides a means of further factoring things out from it.

Performing stack clean && time stack test --fast esqueleto on master and this branch yield a runtime of 2:25 vs 1:47, about 30s of savings.

Incremental builds are even faster. Factoring out the models into a separate module has a considerable benefit on compile-times. Performing a build (stack build --test --no-run-tests --fast esquleto) and then editing Common.Test to have an additional test case on master results in a rebuild that takes 54s of wall time. On this branch, the same change takes 12s to rebuild.

Before submitting your PR, check that you've:

• [ ] Bumped the version number.
• [ ] Documented new APIs with Haddock markup.
• [ ] Added @since declarations to the Haddock.
• [x] 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).