Ink! Set Caller and Callee

test.rs

  let accounts =
                ink_env::test::default_accounts::<ink_env::DefaultEnvironment>();

            assert_eq!(erc20.balance_of(accounts.bob), 0);

            // Set the contract as callee and Bob as caller.
            let contract = ink_env::account_id::<ink_env::DefaultEnvironment>();
            ink_env::test::set_callee::<ink_env::DefaultEnvironment>(contract);
            ink_env::test::set_caller::<ink_env::DefaultEnvironment>(accounts.bob);