Tezt/EVM: fix overflow in expected gas fees

Context

What is says on the tin. NB this is currently not an issue in the tests, but this is purely a result of a separate issue that effective_gas_price is always 1 wei.

See the follow up !11509 (merged) for sorting out the gas price (should be min(21000, tx.max_fee_per_gas) wei):

Manually testing the MR

dune exec tezt/tests/main.exe -- --file evm_rollup.ml

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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