feat(docs): Testing guide and getPrivateStorage method

[spalladino]

Adds a guide on testing dapps. While writing the guide, I had to add a getPrivateStorageAt method; but while reviewing other issues I noted that this method already existed and was deleted on purpose, due to lack of authentication.

I'd like to reincorporate the method, since it's useful for testing, and we already lack authentication for accessing other private information (like just calling view on a function that loads private notes) so we are not opening a new avenue for attack.

• Explain how to use sandbox pre-created accounts instead of creating new ones (depends on Add a utility to get Sandbox accounts wallet in Aztec.js #2002)
• Extract utility function to spin up local node and show how to use it
• Set blockTimestamp to block object, add getBlock method to RPC server, and use it to demo warp usage (extracted to Add timestamp to L2Block object and expose via RPC Server interface #2009)
• Add documentation test suite to CI as yet another e2e
• Add getPrivateStorageAt to cheat codes

Depends on #1991

[PhilWindle]

It all looks fine, just boils down to your thoughts @iAmMichaelConnor on re-introducing this method on the RPC server. We used to have it, then we removed it as you expressed concerns about making private state freely accessible. There is a feeling that being able to read private state directly aids in the dev/testing process.

[spalladino]

Yup, when reading old conversations I saw that that removal came out of a suggestion of mine. But I changed my mind after going through this testing guide.

[PhilWindle]

Suggested change

	Instead of connecting to a local running Sandbox instance, you can also start your own Sandbox within the nodejs process running your tests, for an easier setup. To do this, import the `@aztec/aztec-sandbox` package in your project, and run `createSandbox` during setup. Note that this will still require you to run a local Ethereum node like Anvil locally.
	Instead of connecting to a local running Sandbox instance, you can also start your own Sandbox within the nodejs process running your tests, for an easier setup. To do this, import the `@aztec/aztec-sandbox` package in your project, and run `createSandbox` during setup. Note that this will still require you to run an Ethereum node like Anvil locally.