feat #2007: parallel isolated tests

[leostera]

In this PR I'm trying to isolate the tests from each other with the smallest change possible to how they're executed to allow for:

1. parallel test execution
2. better test caching with dune
3. better test quality by avoiding leaked state across tests
4. fix data-dependencies across tests that we have today (some FIXMEs in newer integration tests)

The goal is to be able to run dune test -w.

Implementation Notes

The interesting bits are in test_utils.ml and in a new support.ml module where case and database_case are two-sides of the same coin:

• Test_utils.case wraps a regular Lwt test in a transaction, starts it and rolls it back after the test has ended.
• Support.database_case does the same but ALSO executes the test.

There's plenty of refactoring to do here, so don't read too much into this. Eventually those 2 functions will merge as we split out different tests groups into their own dune stanzas.

Most of the rest of the changes are just wrapping existing tests in Test_utils.case.

Unfortunately, I'm blocked by a strange SEGFAULT issue that seems to be coming from Caqti :/ – so I'm opening the PR as a draft, and will continue this work next week.