feat(flake): Add unit testing

Change-Id: Idc9bbca0e752b21a5293e2ad4819da9af42ca8ca
Reviewed-on: https://git.clicks.codes/c/Infra/NixFiles/+/782
Tested-by: Skyler Grey <minion@clicks.codes>
Reviewed-by: Samuel Shuert <coded@clicks.codes>
diff --git a/shells/default/default.nix b/shells/default/default.nix
index d7d85e1..d453305 100644
--- a/shells/default/default.nix
+++ b/shells/default/default.nix
@@ -5,6 +5,7 @@
 
 {
   mkShell,
+  nix-unit,
   reuse,
   deploy-rs,
 }:
@@ -12,5 +13,6 @@
   packages = [
     reuse # Used to provide licenses & copyright attribution
     deploy-rs # Used to deploy to our servers
+    nix-unit # Used to do unit testing
   ];
 }