chore: perform startup zkEVM storage migration just before test contract execution

[nbaztec]

What :computer:

• Perform startup storage migration in zksync mode after test contract deployment and just before test contract execution. This allows migrating any storage changes like nonce/balance that are performed before test execution.
• Removes the startup_zk in favor of a more granular migration point.

Why :hand:

• As per #731 we have certain scenarios where it's required that the test contract already be deployed for the sound functioning of our tests and scripts.
• startup_zk was introduced as a quick and dirty solution.

Evidence :camera:

Tests pass

[nbaztec]

We can substitute it with functional and_then but it is actually the idiomatic way. I reckoned adding a 4th invariant (SkipInEvm, EvmModeNoop, etc.) but it didn't feel right in the enum category at all, and was requiring quite a bit of docs in explanation, which is usually a direct sign that it doesn't belong there.