chore: add nixtest for testing and a couple of tests

tests/loader_test.nix

@@ -0,0 +1,57 @@
+{
+  pkgs,
+  ntlib,
+  rensa,
+  ...
+}: {
+  suites."Loader" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "cell sibling access";
+        expected = {
+          hello = "world";
+        };
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          testFlake.x86_64-linux.test.test;
+      }
+      {
+        name = "load file";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          mkdir -p "cells/testcell"
+          echo '{ hello = "world"; }' > "cells/testcell/packages.nix"
+
+          cat > "flake.nix" << 'EOF'
+          {
+            outputs = inputs: let
+              rensa = import ${../lib} { lib = import "${pkgs.path}/lib"; };
+            in rensa.build {
+              inputs = {};
+              cellsFrom = ./cells;
+              cellBlocks = with rensa.blocks; [ (simple "packages") ];
+              systems = [ "x86_64-linux" ];
+            };
+          }
+          EOF
+
+          result=$(nix eval --impure .#x86_64-linux.testcell.packages.hello)
+
+          assert "$result == \"world\"" "should equal to world"
+        '';
+      }
+    ];
+  };
+}