feat(docs): Testing guide and getPrivateStorage method (#1992)

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.

- [x] Explain how to use sandbox pre-created accounts instead of
creating new ones (depends on #2002)
- [x] Extract utility function to spin up local node and show how to use
it
- [x] Set blockTimestamp to block object, add getBlock method to RPC
server, and use it to demo `warp` usage (extracted to
AztecProtocol/aztec-packages#2009)
- [x] Add documentation test suite to CI as yet another e2e
- [x] Add `getPrivateStorageAt` to cheat codes

Depends on #1991

yarn-project/noir-libs/value-note/src/utils.nr

@@ -45,7 +45,7 @@ fn decrement(
     owner: Field,
 ) {
     let sum = decrement_by_at_most(balance, amount, owner);
-    assert(sum == amount);
+    assert(sum == amount, "Balance too low");
 }
 
 // Similar to `decrement`, except that it doesn't fail if the decremented amount is less than max_amount.

yarn-project/noir-libs/value-note/src/value_note.nr

@@ -10,12 +10,14 @@ use dep::aztec::oracle::{
 
 global VALUE_NOTE_LEN: Field = 3; // 3 plus a header.
 
+// docs:start:value-note-def
 struct ValueNote {
     value: Field,
     owner: Field,
     randomness: Field,
     header: NoteHeader,
 }
+// docs:end:value-note-def
 
 impl ValueNote {
     fn new(value: Field, owner: Field) -> Self {