Don't use nix-store --add in tests

[Chris Warburton]

inNixedDir calls 'nix-store --add'. This can cause problems, since the
output doesn't get picked up by the Nix garbage collector as a
reference, i.e. we might have derivation A which depends on B and C,
where C is the result of `nix-store --add`. As long as A is "alive", the
garbage collector won't delete B, but it might still delete C since it
doesn't realise that A needs it.