[Feature]: Allow setting block::timestamp in unit tests

[qalisander]

What is the feature you would like to see?

Could be helpful in such contracts testing such as VestingWallet

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[0xNeshi]

Default value should be same as in Foundry, which is 1.
https://book.getfoundry.sh/reference/config/testing#block_timestamp

We need to handle the timestamp interval (block time), i.e. different transactions in the same test case should not have the same timestamp, even when the initial value is manually set.