Set `dest_store` in the test hydra config, so that the testsuite ensures
that the distinction between the local store and the destination store
is properly taken into account.
Fix#938
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.)
