Fix bugged implementation

system/t07_serve/RootDirInaccessible_gold

@@ -1 +1 @@
-ERROR: Configured rootDir '/rootDir/does/not/exist' is inaccessible, check access rights
+ERROR: Configured rootDir '/root' is inaccessible, check access rights

system/t07_serve/__init__.py

@@ -19,7 +19,10 @@ class RootDirInaccessible(BaseTest):
     """
     fixtureDB = False
     fixturePool = False
-    configOverride = { "rootDir": "/rootDir/does/not/exist" }
+
+    configOverride = {
+        "rootDir": "/root" # any directory that exists but is not writable
+    }
     runCmd = "aptly serve -listen=127.0.0.1:8765"
 
     def run(self):