tests: reduce unnecessary rebuilds of several tests

Some test were referring to the whole source code via ${self} which amde them rebuild on every single commit.

This is not mitigated by introduceing `self.filter { include = [...]; }` allowin to a content addressed subset of the source code in tests.

lib/flake-module.nix

@@ -4,8 +4,14 @@
   self,
   ...
 }:
+let
+  inherit (lib)
+    filter
+    pathExists
+    ;
+in
 {
-  imports = [
+  imports = filter pathExists [
     ./jsonschema/flake-module.nix
     ./inventory/flake-module.nix
     ./build-clan/flake-module.nix