The fee cheatcode will set the block.basefee.
// Obtain our cheat code contract reference.
IStdCheats cheats = CheatCodes(0x7109709ECfa91a80626fF3989D68f67F5b1DD12D);
// Change value and verify.
cheats.fee(7);
assert(block.basefee == 7);function fee(uint256) external;