nar-archive.cc: add tests for the nar index

tests/nar-index.nix

@@ -0,0 +1,23 @@
+with import ./config.nix;
+
+rec {
+    a = mkDerivation {
+        name = "nar-index-a";
+        builder = builtins.toFile "builder.sh"
+      ''
+        mkdir $out
+        mkdir $out/foo
+        touch $out/foo-x
+        touch $out/foo/bar
+        touch $out/foo/baz
+        touch $out/qux
+        mkdir $out/zyx
+
+        cat >$out/foo/data <<EOF
+        lasjdöaxnasd
+asdom 12398
+ä"§Æẞ¢«»”alsd
+EOF
+      '';
+    };
+}