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`
This replaces the '(...)' installable syntax, which is not very
discoverable. The downside is that you can't have multiple expressions
or mix expressions and other installables.
This currently fails because we're using POSIX file locks. So when the
garbage collector opens and closes its own temproots file, it causes
the lock to be released and then deleted by another GC instance.
