Move sandbox friendly temporary store from `-remote` to `-tests`

[sorki]

So it can be reused by hnix and nix-diff. Related to haskell-nix/hnix#903 and Gabriella439/nix-diff#79 (comment)

[sorki]

It also needs some improvements as the nix-daemon cannot chown store to gid 30000 - error: changing ownership of path '/run/user/1000/test-nix-store-df5003dd48cfc52f/store': Invalid argument and this is making the test output look bad.