Tidy test data scripts

[peterdudfield]

Detailed Description

Currently there are a few locations for making the test data, would be goo to ahve these all in once place

1. https://github.com/openclimatefix/nowcasting_dataset/blob/main/scripts/generate_test_data.py
2. https://github.com/openclimatefix/nowcasting_dataset/blob/main/tests/data_sources/get_test_data.py

Context

If the raw data gets updated, its nice to have quick and easy ways to make test data

Possible Implementation

make a folder in scripts called tests_data?

[JackKelly]

I really like the idea of having a folder called scripts/test_data or similar! As the number of scripts has grown, it's become harder to see which scripts are for generating test data, and which are for generating "real" data.

I'd maybe suggest calling the directory something like scripts/generate_data_for_tests just to make it really explicit that the folder doesn't hold the actual data itself, but instead holds the scripts for generating the data.