Keep testnet access nodes for older time frames

After a spork, it is no longer possible to access block or transaction data from a time frame before the spork. This is not a problem for mainnet, as there are documented URLs for the older access nodes specified here: Past Spork Info - Flow Documentation

Sadly, there are no similar nodes for the testnet.

Due to the way the flow chain servers shutdown before a spork, sometimes there is no time to scrape the last blocks before the shutdown. If we execute a transaction just before the spork, and don’t get a chance to read it, then we have no idea if it succeeded or not. My plan is to update our code to try to read this transaction data using URL from an earlier access node.

To aid testing of chain crawling code, it would be nice to be able to access at least the previous timeframe on the testnet. Then I would have more confidence that it will work on the mainnet in our production environment.