L1->L2 testing can be simplified

[FabijanC]

Problem

To test L1->L2 messages coming from Anvil, our L1L2 example contract (defined in L1L2.sol, compiled in L1L2Example.json), we must:

1. increment balance on L2
2. withdraw from L2 to L1
3. deposit from L1 back to L2.

Deposit cannot be called right away because there is a constraint in the code which otherwise leads to reversion:

starknet-devnet-rs/contracts/l1-l2-messaging/solidity/src/L1L2.sol

Line 62 in eb8d800

require(amount <= userBalances[user], "The user's balance is not large enough.");

Proposal

One of the following can be done:

a. Create a common function in our Rust tests which abstracts the three steps described above.
b. Add a new method to L1L2.sol without the require on balance; could have one of the following names:

• mockDeposit
• forceDeposit

c. Add a new example contract with the method described in b.