tests: fix `getOutput` on stubs

Setting `outputSpecified` prevents `getOutput` from descending into
outputs, which don't have an overridden `outPath`.

Additionally, use `__spliced` to permit derivations to use the dummy as
an input.

tests/modules/programs/autojump/default-settings.nix

@@ -8,6 +8,7 @@ with lib;
 
     test.stubs.autojump = {
       buildScript = "mkdir -p $out/bin; touch $out/bin/autojump";
+      outPath = null;
     };
 
     nmt.script = ''

tests/modules/services/git-sync/basic.nix

@@ -24,7 +24,7 @@
         WantedBy=default.target
 
         [Service]
-        Environment=PATH=/nix/store/00000000000000000000000000000000-openssh/bin:/nix/store/00000000000000000000000000000000-git/bin
+        Environment=PATH=@openssh@/bin:/nix/store/00000000000000000000000000000000-git/bin
         Environment=GIT_SYNC_DIRECTORY=/a/path
         Environment=GIT_SYNC_COMMAND=@git-sync@/bin/git-sync
         Environment=GIT_SYNC_REPOSITORY=git+ssh://user@example.com:/~user/path/to/repo.git

tests/stubs.nix

@@ -41,8 +41,18 @@ let
         dummyPackage
       else
         pkgs.runCommandLocal name { pname = name; } buildScript;
-    in pkg // optionalAttrs (outPath != null) { inherit outPath; }
-    // optionalAttrs (version != null) { inherit version; };
+    in pkg // optionalAttrs (outPath != null) {
+      inherit outPath;
+
+      # Prevent getOutput from descending into outputs
+      outputSpecified = true;
+
+      # Allow the original package to be used in derivation inputs
+      __spliced = {
+        buildHost = pkg;
+        hostTarget = pkg;
+      };
+    } // optionalAttrs (version != null) { inherit version; };
 
 in {
   options.test.stubs = mkOption {