add simple override test

Signed-off-by: Markus Theil <theil.markus@gmail.com>

test.nix

@@ -22,7 +22,8 @@ let
   '';
 
   inherit (pkgs) lib;
-  # Generate a git repository hat can be served via HTTP.
+
+  # Generate a git repository that can be served via HTTP.
   #
   # By default the repository will contain an empty `test.txt`
   # file. For all defined tags the name of the tag is written to that
@@ -423,4 +424,17 @@ in
           '';
         };
     };
+
+  gitDependencyOverride = mkGitTest rec {
+    name = "git-dependency-override";
+    repositories."foo" = gitRepo;
+    commands = ''
+      npins init --bare
+      npins add git http://localhost:8000/foo -b test-branch
+      npins show
+
+      OUTPATH=$(NPINS_OVERRIDE_foo=/foo nix-instantiate --eval npins -A foo.outPath --impure)
+      eq "$OUTPATH" /foo
+    '';
+  };
 }