In iossim, use the user's home directory if no override is given.

testing/iossim/iossim.mm

Index: testing/iossim/iossim.mm
diff --git a/testing/iossim/iossim.mm b/testing/iossim/iossim.mm
index b5cbd51f5ff0a0c5cac4e0938fca9a8057cd2aff..6fda2ca4c2a49ba5be673f971157453550ca1663 100644
--- a/testing/iossim/iossim.mm
+++ b/testing/iossim/iossim.mm
@@ -914,6 +914,8 @@ int main(int argc, char* const argv[]) {
                simHomePath);
       exit(kExitInitializationFailure);
     }
+  } else {
+    simHomePath = NSHomeDirectory();
   }
 
   // Create the config and simulator session.