pythonCatchConflictsHook: add test for multiple dependency chains

Add a test where a conflicting package can be found at the end of
multiple dependency chains. This is far too simple an example to
demonstrate the ill effects of exponential time complexity, but does
serve to demonstrate how the error output changes when each path is only
visited once.

pkgs/development/interpreters/python/hooks/python-catch-conflicts-hook-tests.nix

@@ -143,4 +143,46 @@ in {
     };
   in
     expectFailure toplevel "Found duplicated packages in closure for dependency 'leaf'";
+
+  /*
+    Transitive conflict with multiple dependency chains leading to the
+    conflicting package.
+
+    Test sets up this dependency tree:
+
+      toplevel
+      ├── dep1
+      │   └── leaf
+      ├── dep2
+      │   └── leaf
+      └── dep3
+          └── leaf (customized version -> conflicting)
+  */
+  catches-conflict-multiple-chains = let
+    # package depending on dependency1, dependency2 and dependency3
+    toplevel = generatePythonPackage {
+      pname = "catches-conflict-multiple-chains";
+      propagatedBuildInputs = [ dep1 dep2 dep3 ];
+    };
+    # dep1 package depending on leaf
+    dep1 = generatePythonPackage {
+      pname = "dependency1";
+      propagatedBuildInputs = [ leaf ];
+    };
+    # dep2 package depending on leaf
+    dep2 = generatePythonPackage {
+      pname = "dependency2";
+      propagatedBuildInputs = [ leaf ];
+    };
+    # dep3 package depending on conflicting version of leaf
+    dep3 = generatePythonPackage {
+      pname = "dependency3";
+      propagatedBuildInputs = [ (customize leaf) ];
+    };
+    # some leaf package
+    leaf = generatePythonPackage {
+      pname = "leaf";
+    };
+  in
+    expectFailure toplevel "Found duplicated packages in closure for dependency 'leaf'";
 }