When `NIX_DAEMON_PACKAGE` is set, make all the tests use the Nix daemon.
That way we can test every piece of Nix functionality both with and
without the daemon.
Tests for which using the daemon isn’t possible or doesn’t make sens can
selectively be disabled with `needLocalStore`
Requires a slight update to the test infra to work properly, but
having the possibility to group tests that way makes the whole thing
quite cleaner imho