fix: add support for SELFBALANCE opcode

[Karrq]

Motivation

When an account balance is read, and zk mode is enabled, we intercept the BALANCE opcode and read the balance from the L2 Eth system contract.
When checking the test address's balance (via address(this).balance), any balance change seems to not be reflected, causing failures on some tests.

Solution

address(this).balance doesn't always use the BALANCE opcode, but the SELFBALANCE opcode instead (or at least, it can use that opcode), as it is more gas efficient.
Adding support for this opcode ensures that this pattern reads the expected contract balance.