feat(anvil): support mining with same block.timestamp

[yash-atreya]

Motivation

Closes #9142 + Closes #7931

Solution

Replace the check next_timestamp <= last_timestamp with next_timestamp < last_timestamp.

Anvil can now mine blocks with the same timestamp as the previous block timestamp. This aids in running forks or testing chains that have sub-second block times, wherein block.timestamp stays constant until exactly one second has passed.

Added relevant tests.

[yash-atreya]

@mattsse this effectively undos the fix for #6096. See here:

foundry/crates/anvil/tests/it/anvil_api.rs

Line 611 in 0d9d8a5

assert!(block.header.timestamp >= latest_block.header.timestamp);

.

If users expect the next_block_timestamp after revert to be always greater than block.timestamp the snapshot was taken at, we can probably just add 1s here

foundry/crates/anvil/src/eth/backend/mem/mod.rs

Line 834 in 2044fae

let reset_time = block.header.timestamp;

while resetting. wdyt?