First use tweaks

[ThinkChaos]

Hi, thanks for building this and haumea, both are cool projects!

Here's a couple changes I did when getting setup. If you'd like me to split this in any way, let me know.
All changes are pretty small though.

1. fix: interpret path printed by nix as relative to repo root

   This was causing check to always see the tests as new/failing, but
   review to show a diff with no changes.

   My setup is similar to the subflake template, so it's probably worth adding automated tests for the templates as that would've probably caught this bug.

2. feat(check): track test additions separately from failures

   Tweak the output so those cases are more easily distinguishable,
   especially the color used by the + symbol next to the test name.
   Exit code is now 2 if all existing tests succeeded, but some were added.

   Here's an example output in the common case:
   
   Here's a comparison of "failing and added state" which is what motivated me to change this:
   
   I was seeing the above a lot because of 1.
   

3. fix: only show flake and dir warning when relevant

   Forgetting to set src was triggering it.
   Ideally we'd just change the function to use { src, ... }@args: so Nix
   can provide an explicit error the users are accustomed to.

[figsoda]

Sorry for the super late review

1: I can't reproduce this error. Do you have the snapshots added to git? If this fixes the issue, maybe namaka can automatically add the snapshots to git (like nix does with flake.lock)?

Everything else lgtm, I'm happy to revert (1) and merge everything else as-is.