Add `--test` flag

[crabdancing]

Sometimes configurations should be built and deployed, but not set to the default new configuration on reboot. nixos-rebuild test is important for testing functionality on the system.

Accordingly, there should be a --test flag to do the equivalent with deploy.

[cole-h]

I implemented this in #353. It seems to work for me. Please take a look!

[crabdancing]

I actually built my own tool since I had a bunch of specific things I wanted. :P I'm sorry to say I'm no longer in a position to try it. That said, your change looks good. ^w^

[cole-h]

It might be worth it to keep this open until that PR is merged for all the other folk who were wanting this 😅