Nowadays `Builds` doesn't reference `Project` directly anymore. This
means that simply resolving both `jobset` and `project` with a single
JOIN from `Builds` doesn't work anymore. Instead we need to resolve the
relation to `jobset` first and then the relation to `project`.
For similar fixes see e.g. c7c4759600.
By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.
(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)