Move dedicated `use`s into `mainnet_blocks_from_rpc`

[hai-rise]

To remove these warnings when running cargo test (without the rpc-storage feature) in main:

warning: unused imports: `ProviderBuilder` and `Provider`
 --> crates/pevm/tests/mainnet.rs:3:22
  |
3 | use alloy_provider::{Provider, ProviderBuilder};
  |                      ^^^^^^^^  ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused imports: `BlockId` and `BlockTransactionsKind`
 --> crates/pevm/tests/mainnet.rs:4:23
  |
4 | use alloy_rpc_types::{BlockId, BlockTransactionsKind};
  |                       ^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^

warning: unused import: `reqwest::Url`
 --> crates/pevm/tests/mainnet.rs:5:5
  |
5 | use reqwest::Url;
  |     ^^^^^^^^^^^^

warning: unused import: `PevmChain`
 --> crates/pevm/tests/mainnet.rs:7:19
  |
7 | use pevm::chain::{PevmChain, PevmEthereum};
  |                   ^^^^^^^^^