Remove dummy model use when testing forecasting with PV sites

[Sukh-P]

For pv sites we currently aren't testing against our actual models and instead are using the dummy model https://github.com/openclimatefix/india-forecast-app/blob/main/tests/test_app.py#L81 which generates fake predictions, I imagine this was done previously when pv models weren't quite ready but perhaps now it feels like the right time to remove this to improve our model test coverage.

A wider question around whether the dummy model should be kept in, could be useful for other mocking/testing.

[Dakshbir]

Could you assign this issue to me?
I would love to contribute :)